If you ain't played Agario yet, I insist you try. It is the single most silly idea for an MMO ever. But it's fun, it's addictive, and it's free so... Give it a shot. Just be careful and don't play for four hours straight like I did.
I burnt out on Rumble World really quickly. It works on a handheld better than on a console, but you can only make a Diablo-lite so simple before it becomes a game of repetitive button mashing. Got four hours out of it, though. That's more than I could say for most FTP games.
Screw you for bringing me to the pointless yet addicting game of Agario. This game is way to fun for something so simple.