Diep.io is another classic game, this time with players controlling tanks, destroying obstacles, and enemy tanks to gather points and level up their own machines. A reasonably involved upgrade system lets you outfit your tank to your liking, allowing you to build bullet-hell monstrosities, agile attackers, or powerful snipers. 

Varied game modes, from free-for-alls to team battles and more give Diep.io some good replay value and reasonable depth, while still being a pretty accessible browser game.