Agar.io: I dislike this game. It's not fun for me. I always get cornered, or I always get eaten by someone shooting a part of their cell at me. It's not easy for the newbies, in my opinion.
Slither.io: Played it one time, disliked it as well. It's too easy to just get surrounded by others (not to mention zooming is a thing) and it's not my type of game. I just felt the map was too large, and I was too small a bunch of the time.
Diep.Io: A rage-inducing game (They all are, if you think about it...), but my favorite of the bunch. Upgrades + Stats allow players to decide which playstyle they'd like to go with; whether it be controlling units, a big cannon, or having a tri-shot. Probably my favorite due to the strategies. The cannon though might be a little bit OP, but I've got a strategy for those guys. :) (Still trying to get the Necromancer Upgrade so I can see what it does.) It's a pain when you die, though.
So, what do you guys think of these browser-games? What's your Favorite? Your Least Favorite?