Vainglory is Super Evil Megafun
Vainglory. I've been playing it for a bit over a month now, and it's great. I hadn't actually tried any of the previous games that were called multiplayer online battle arenas (MOBA), so I didn't really know what to expect, but it's basically a combat videogame played like a team sport, with a variety of characters, upgrades and items.
The thing that it reminds me the most of is the feeling of playing Quake CTF -- multiplayer team-based, goal-based action where playing as a team can really make a big difference. But it's not a first-person shooter, it's top-down, controlled a little like an RTS game, but on the iPad (or iPhone, if you really want), with a single unit that you need to control, no army-building and base-building to do.
You play games which last 10-40 minutes, most of them being 20-30. That's long enough to make it challenging but short enough that I can actually find the time to play games fairly regularly.
I'm loving it. It's the first game in years that sucked me in as this hard and this long, and easily the most addictive of the mobile games I've played.
You should try. And then you should get in touch and tell me who you are on Vainglory so I can add you as a friend.