Got to give it to Knights of Pen and Paper +1. I got so much out of the game.
I have to give a shout out to Swordigo here.
Sometimes I can't believe how well the touchscreen controls work for that game. For sure in my top 5, though that could change once I actually get around to beating it. Also Link for the game. It really deserves the praise it receives.
But good heavens what a challenge! And great controller support to boot!
For me it's definitely Wazhack, I've put so many hours into it.
Both the Knights of Pen & Paper games rank really high on my favorites list. Also have to send a little love towards Organ Trail, and agar.io has been a huge time killer lately too now that they've got an official mobile app.
My favorite of all time though is a game that is Dungeon Defenders: Second Wave. Unfortunately it's no longer on the Google Play store, but it can be found if you're really looking for it. Way back when the original Dungeon Defenders was new on the PS3, my buddy picked up the demo when he was staying at my place and we played the demo for 3+ hours (there was only ~30 minutes of content in the demo). He bought the game and I was addicted. The only problem was that I could only play when he was around with his PS3 because I didn't have one. I found Dungeon Defenders: Second Wave on the app store and immediately bought it as it was a complete mobile version of the game I was in love with. Sure, the controls were a bit hard to get used to, but once I had those down, it was essentially the same game. I drained my battery countless times with that game.