NOVA is such a pain to play. I don't know, the controls just don't do it for me. Maybe it's because I'm playing it on a Palm Pre? Who knows. Touch-screen controls for games based heavily on reflexes are usually a pain to play, which is why I think that without buttons, mobile phone gaming (iOS, Android, webOS) will always sort of be relegated to that 'casual' status.
Anyway, as for iOS games, I've got quite a bunch on my old-ass iPod touch, but one of my favorites is Gamevil's Baseball Superstars 2009. I've got 2010—and I haven't played it that much—but I spent a lot of time with 2009 and it's just phenomenal. I'm not even a big fan of sports games, but I've literally played the game for hours on end.
Helsing's Fire is a great, unique puzzle game with a lot of style.
Sword and Poker is another great one; a turn-based dungeon crawler built around a poker-oriented battle system.
I do like Square Enix's offerings too, like Vanguard Storm and Hills and Rivers Remain; I like it when they put something new out instead of porting old games like Final Fantasy. I haven't tried Chaos Rings; I think my poor first-gen iPod would buckle and break.
We can't have a thread about iOS games without mentioning Canabalt, the run'n'jump game with music by our very own Danny B.