Tiers

In this instance, the Tank is seen as Tier 1, according to the Diep.io changelog.

Before August 2nd the wikia had it wrong, we assumed Level 15 was Tier 1, Level 30 was Tier 2, and Level 45 was Tier 3. But according to a changelog post, the Sprayer was made 'Tier 4'. This means that the whole time, Level 45 was Tier 4 and we were just assuming. Now it is confirmed, and all info here is correct.

Tier 2: Level 15
Upon reaching Level 15, the player may select one of these Tier 2 specializations but the background color of the tank, in class selection, represents the base stat debuffs given to the class to compensate for it.

Twin: Tier 2 Class upgrade that results in gaining 2 shorter barrels. The bullet damage and movement speed decreases, and the spread is slightly bigger.

Flank Guard: Tier 2 Class upgrade that results in gaining another barrel on the side opposite to the existing one. It alternates between shooting forward and backward. The smaller flank barrel fires at half the rate of the normal one. The flank guard gets a debuff to health regeneration

Machine Gun: Tier 2 Class upgrade that results in an improved rate of fire, and a slight spread to the shots. Like the Twin, its bullet damage also passively decreases.

Sniper: Tier 2 Class upgrade that slightly increases the player's field of view and increases bullet speed/penetration at the cost of drastically decreasing reload time.

Tier 3: Level 30
Upon reaching Level 30, the player may select one of these Tier 3 specializations but the background color of the tank, in class selection, represents the base stat debuffs given to the class to compensate for it.

Twin Upgrades
Triple Shot: A triple barreled gun, but each turret is 45 degrees apart from the last, forming a wide spread that has little focus of fire. Can upgrade to Triplet or Penta Shot at Level 45. Triple Shot gets a base stat debuff to movement speed

Twin Flank: Will add two barrels to the back, and act like the Flank Guard above, respectively, but with two barrels on each side and gets a base stat debuff to bullet damage and movement speed.Can upgrade to the Octo Tank or Triple Twin at Level 45.

Quad Tank: Often helpful for farming and hitting multiple enemies, but hard to aim with and has easily exploitable holes in its defense unless you are spinning or in auto spin. Quad tank gets a base stat debuff to movement speed. Each turret is spaced out at 90 degree angles, making it more like the flank guard than the twin. Can upgrade to Octo Tank at Level 45.

Machine Gun Upgrades
Gunner: The Gunner is an upgrade from the Machine Gun. The Gunner gets 4 cannons, each relatively small, and so are the bullets it shoots. The Gunner's bullet damage and penetration are decreased due to bullet shape and reload is debuffed due to base stat debuff compensation(since it has alot of guns already). This allows for hard-to-escape bullet storms that can overwhelm and destroy other tanks with ease. Can upgrade into the Auto Gunner or Auto Tank.

Destroyer: A long-range cannon that fires extremely slowly, but hits incredibly hard if you play your Skill Points right. The gun has such a forceful shot, the recoil sends you flying fast back a great distance. Damage and Health are passively improved with the upgrade. Can upgrade to Hybrid at Level 45. Quick tip: If you are going to use this class and you are trying to move around, shoot behind you to move faster in the other direction so that you can make up for it's class base stat debuff to movement speed.

Sniper Upgrades
Assassin: Increases field of vision, damage and bullet speed further. This class gets a base stat debuff to movement speed. Simply an improved Sniper. Can upgrade to Stalker or Ranger at Level 45.

Overseer: Instead of firing bullets, the Overseer summons controllable Triangles called drones that follow your mouse when you hold left click (or pressing E to autofire), allowing you to swarm enemies with a horde of minions, and you can right click to have them spread away from your mouse. The triangles will automatically attack any polygons close, however. The overseer can have a max of 8 Triangles. When not moving, the Triangles will circle around you forming a pseudo-shield. Reload helps spawn minions faster when not at max capacity. Bullet penetration and bullet speed turns into drone health and drone speed.This class gets a base stat debuff to reload. Can upgrade to Necromancer, Manager, Overlord or Overtrapper at Level 45.

Hunter: The Hunter shoots a bullet, followed by a larger bullet, each one being shoot out of a specific barrel. The Hunter seems to add some extra damage and penetration, but shoots very slowly. This class gets a base stat debuff too. Can upgrade to X Hunter or Predator at Level 45. Note: It is the class that was most changed during the constant updates, it was nerfed right after it was released (shooting a burst of 7 mini bullets) and later buffed to its original firing concept.

Trapper: Has the ability to place triangle-shaped traps, where when tanks touch these traps, they will take damage. You can spawn a limited but large number of these traps. This class gets a base stat debuff to health regeneration. The Trapper can upgrade to the Tri-Trapper, Mega Trapper, Gunner Trapper, or the Overtrapper at level 45.

Flank Guard Upgrades
Twin Flank: Think of the regular Flank Guard, but with twice as many barrels. There are 2 in front, and 2 behind your tank.This class gets a base stat debuff to damage. Can upgrade into the Octo Tank or Triple Twin at Level 45. Also, the back guns are now exactly the same size as the front ones.

Quad Tank: Imagine the Flank Guard, but double the barrels, one on all four sides shooting directly, often helpful for farming and hitting multiple enemies, but hard to aim with and has easily exploitable holes in its defense.This class gets a base stat debuff to movement speed. Can upgrade to Octo Tank at Level 45. Not recommended to be used, as mentioned above, very exploitable weaknesses that are easy to take advantage of.

Tri-Angle: Think of a regular tank with 2 barrels at the back forming, as the name implies, a triangle. The barrels at the back give it a decent speed boost when going forward, while providing a possible bullet shield on the front. Great for chasing and/or avoiding tanks, as it is one of the fastest tanks in the game. This class gets a slight base stat debuff to reload.

Unnamed Flank Guard Upgrade: This tank has 3 Automatically shooting turrets. You lose control of your tank,but, however, you can still move around.

Standard Tank Upgrades
Smasher: A unique class of tank that can only be accessed by leveling the default tank to level 30. This tank lacks a cannon, and by extension bullets to shoot. Instead, this tank relies entirely on body damage, withstanding more bullets and knock back to be able to get in close and ram other tanks and polygons. Bullet Speed, Bullet Penetration, Bullet Damage, and Reload upgrades do not affect this tank in any way, and are thereby removed from the upgrade window. This class gets a base stat nullification to all bullet-related upgrades, as mentioned above.

Tier 4: Level 45
Every class may upgrade to these Tier 4 specializations. The background color of the tank, in class selection, represents the base stat debuffs given to the class to compensate for it.

Twin Flank Upgrades
Octo Tank: Upgraded from Twin Flank and Quad Tank. Fires in all eight normally used directions (i.e the cardinals and diagonals). This class gets a base stat debuff to movement speed.

Triple Twin: An upgrade from Twin Flank that now has three twin barrels and is consistently seen at the top of the leader board. It's got more bullet concentration but a bit of more vulnerable than Octo Tank. This class gets a base stat debuff to reload.

Quad Tank Upgrade
Octo Tank: Upgraded from Twin Flank and Quad Tank. Fires in all eight normally used directions (i.e the cardinals and diagonals). This class gets a base stat debuff to movement speed.

Triple Shot Upgrades
Triplet: An upgrade from Triple Shot. A triple barreled tank, that focuses fire in one direction. Looks like original twin shot, but has a longer gun between the two outer guns. The bullet damage will decrease a bit more. Greatest DPS frontal power. This class gets a base stat debuff to movement speed.

Penta Shot: An upgrade from Triple Shot. Shoots five bullets evenly spread out in a more manageable angle than the Triple Shot. With a passively higher penetration, nothing can reach you. Also, retreating is easy because there is great recoil when firing. This class gets a base stat debuff to reload.

Overseer Upgrades
Overlord: An upgrade to the Overseer, the Overlord can summon two extra minions and spawns them faster. Other than that, nothing is new. This and the Necromancer also have a secondary attack used by right-clicking that send all minions traveling away from your mouse. This class gets a base stat debuff to movement speed.

Necromancer: An upgrade from the Overseer. Upon touching or killing a Yellow Square, it becomes your minion, and you can use it to quickly amass a huge army of up to 34 squares, however, you must have max Drone Count stats. Be wary however, never enter the Pentagon Nest, as you will run out of Squares and become vulnerable quickly.

Manager: An upgrade from the Overseer. This is a mixture of Overseer and Stalker features, which has only one of Overseer's barrel instead of two, and also has the ability to go invisible like Stalker does. As a note your minions will not go invisible with you, and if not directed, will circle around you, but you can move them and stay invisible as long you don't move. Great for camping unnoticed. This class gets a base stat debuff to drone damage.

Overtrapper: An upgrade from the Trapper and Overseer. This upgrade adds two Drone spawners to the back akin to the Tri-Angle. These drones are similar to the Hybrid in that they will attack nearby polygons and players but cannot be controlled. This class gets a base stat debuff to Bullet Damage.

Assassin Upgrades
Stalker: An upgrade from the Assassin. This upgrade increases your range and field of view. When you are still you become invisible. Stealth is broken when you shoot or move. This is the best class if you like the classic sniper feel with extra benefits. However the damage seems to be decreased but this is not confirmed. This class gets a base stat debuff to movement speed.

Ranger: An upgrade from the Assassin. It increases your range and field of view even more. That's the only feature to the class (so far). Great for dodging and predicting bullet patterns.This class gets a base stat debuff to movement speed.

Tri-Angle Upgrades
Fighter: An upgrade from the Tri-Angle. This upgrade allows you to have 2 extra barrels at the sides of your tank. Also passively increases bullet damage. Frontal cannon still does the most damage, but crowd control is a nice addition. This class gets a base stat debuff to reload.

Booster: An upgrade from the Tri-Angle. This upgrade adds 2 extra barrels, 1 additional barrel under each barrel already on the back of your tank. Notice that with full reload, you don't even need to use the arrow keys or 'WASD' keys to move anymore as it has a very noticeable amount of force going from the back. Avoid going for Alpha pentagons as you may be pushed towards it. This class gets a base stat debuff to movement speed.

Destroyer Upgrade
Hybrid: An upgrade from the Destroyer. This upgrade adds a trapezoid-shaped barrel at the back, which is similar to the one from an Overseer class, and it can summon up to 2 drones. The drones cannot be controlled like they are as an Overseer class, instead they will destroy any polygons which are nearby, or home in on the nearest-to-be-seen enemy tank. Otherwise they will go around you as a pseudo-shield. This class gets a base stat debuff to movement speed.

Hunter Upgrade
X Hunter: An upgrade from the Hunter. This upgrade adds an extra barrel to the existing 2 barrels that the Hunter has. It shoots a large bullet followed by a medium-sized bullet and a small bullet. This class gets a base stat debuff to movement speed.

Predator: An upgrade from the Hunter. This upgrade adds an ability to concentrate only at the cursor's present aiming point when holding Right click or Shift key. Note that the concentrating screen DOES NOT move when the tank moves, and the other sides' concentration is reduced, therefore it might be tricky and dangerous for you while doing the aiming since your tank can be off-screen. This class gets a base stat debuff to reload.

Trapper Upgrade
Tri-Trapper: An upgrade from the Trapper. This upgrade adds two additional Trap barrels, increases the fire rate, and decreases the Trap lifespan. This class gets a base stat debuff to movement speed.

Mega Trapper: An upgrade from the Trapper. This upgrade widens the barrel and upgrades the generic traps into larger, slower, shorter lived, and more damaging traps similar to how the Destroyer increases the size of the normal Bullet. This class gets a base stat debuff to Health Regeneration.

Gunner Trapper: An upgrade from the Trapper. This upgrade adds two small cannons to the back of the tank, allowing it to be used both offensively and defensively to great effect. This class gets a base stat debuff to reload.

Overtrapper: An upgrade from the Trapper and Overseer. This upgrade adds two Drone spawners to the back akin to the Tri-Angle. These drones are similar to the Hybrid in that they will attack nearby polygons and players but cannot be controlled. This class gets a base stat debuff to bullet damage.

Smasher Upgrades
Mega Smasher: An upgrade to the Smasher that gives it extra Body Damage, with a slight increase in health. This class gets a base stat debuff to movement speed.

Landmine: An upgrade to the Smasher identical to the Smasher itself. Standing still renders the tank invisible, allowing unsuspecting tanks to run into it and take massive body damage. This class doesn't get any base stat debuffs, despite the color.

Machine Gun Upgrades
Sprayer : The sprayer has an extra rectangular cannon added similar to the Hunter. Its RoF is increased, while the bullet damage is slightly decreased, but its hail of bullets might be every tank's worst nightmare. It's even slower than the Machine Gun, but has even more spread and bullets fired at a time. Extremely good at swarming enemies like Overseers (with high Bullet Damage & Penetration), but weak against other bullet spammers and Sprayers. Good at helping to capture Dominators. Only tank in game that upgrades directly from a Level 15 upgrade to a Level 45 upgrade. This tank gets a base stat debuff to  movement speed.

Gunner Upgrades
Auto Gunner: The Auto Gunner gains an AI turret on it's center. That turret shoots at the closest player in it's FoV. The bullets shot by the auto-turret are larger than Gunner bullets.

Auto Tank: The Auto tank features a circular body with five turrets at 5 sides of the tank. These turrets slowly move from side to side when idle, and tend to 'flop around' when turning very rapidly. Each turret has a 72° field of view, meaning only 2-3 turrets can fire at a single enemy.

Polls
Add polls about classes here. Which tank is the best? Overlord Necromancer Manager Overtrapper

Should the smasher be in Diep.io? Yes, It's amazing! NO! It's terrible! It's a meh.

What Is Your Favorite Of These Tanks? Triple Twin Octo Tank Penta Shot

What Is Your Favorite Of These Tanks? Tri-Angle Booster Fighter

What Is Your Favorite Of These Tanks? Sprayer Predator X Hunter Hunter

Is the Necromancer Over-Powered or Under-Powered? Over powered Under powered It depends how I'm using it.

Favourite Trapper? Trapper Tri-Trapper Mega Trapper Gunner Trapper Overtrapper

is the triplet or gunner trapper more powerful, in your opinion? (poll made by Dabus) triplet gunner trapper both...

Triplet or Sprayer? Triplet Sprayer

Sniper or Tank? Sniper Tank

how big is a lv45 tank? (poll made by Dabus) 10-75 px 76-125 px 126-175 px 176-250 px 251 px or more

Smasher, Mega Smasher or Landmine? Smasher Mega Smasher Landmine

Hybrid Destroyer or Hybrid Trapper? Hybrid Overtrapper

Should the Smasher be a secret class? Yes No

Should the Sprayer be a secret class? Yes No

Should Smasher get another upgrade? Yes (with guns) Yes (with more HP) Yes (faster) Yes (smaller size) No

Which is your favorite?

Auto Gunner

Auto Tank

Which ram branch do you like better?

Tri-Angle

Destroyer

Smasher