Diep.io is a game that is semi-recent and has become very popular. It has a unique concept and I really like the level-up mechanism, and how you can upgrade your tank to many different types of tanks. It's a good concept, but honestly, it just doesn't work for me.
Slither.io is a game that caters to everyone, big or small, and you can win no matter your size - just get a snake to run into you, and that snake's dead, even if you're smaller or bigger than them. I like this concept even more than the agar.io or diep.io concept, because it doesn't favor bigger players even more. It treats everyone equally, instead of like "o u be big, u get to be praktikly ovar powared".
However, diep.io is a game that can be quite chaotic for you unless you don't start leveling up. I often find it difficult to escape a massive bullet attack at right when I spawn, which is why I try to upgrade my Movement speed first. It highly favors leveled up players, and you can find it hard to even get on the leaderboard. I suggest that there be an update, like, for example, you're invincible when you spawn, until you shoot a single bullet.
Diep.io right now just feels like an unevolved version of slither.io. It needs balancing.