One of the more action-packed options, the defining characteristic of Diep.io is tanks. Rather than using your physical body to attack (although it is an option), your main source of offense is a hail of bullets.
The other thing that makes this game stand out is that there are some RPG elements to the upgrade system, in that you can choose different stats to invest in. This element really shines at levels in intervals of 15, when you choose a new major upgrade for your tank, like twin shot or a machine gun.
On the downside, one element this game is missing is the ability to take down much larger opponents. If you're vastly outleveled by an enemy, your only real option is to run and hope they haven't invested heavily in their speed. Or that they decide not to bother with you.