I think Halo: Spartan Assault will take this crown once it releases myself. However, the best I think I've gotten so far is Asphalt 7. The Achievements are icing on the cake of a really nice, solid racer. Plenty of races, plenty of race modes, many cars and mods to unlock. Its races are short, meaning you can easily get one downed when you have a few minutes between tasks in a given day. Others like Skulls of the Shogun take too long to make a great purely-mobile experience. Needing 10+ minutes to fully beat a level pushes the versatility of a mobile game, in my opinion. I like Skulls, I just think it takes longer to beat a level than I'd like, because my recent mobile gaming habits are just a quick 10 minutes before bed (right now, it's Tiki Towers). Many of the popular games don't offer that.