I’m not a huge iOS gamer, but when I do I gravitate toward word games. One of the best in the genre is David Gage’s SpellTower. It’s fun, simple, and has four game variations to keep things interesting. Works well on both iPhone and iPad, and there’s Bluetooth connectivity included for competitive multiplayer.
It’s on sale right now for a buck only for the next 24 hours, so go get it (Cool web site as well.)