Peggle for the iPhone
I like Peggle for the iPhone. A lot of it is that it's a fun game. But it's also a genuinely nice iPhone port of the thing. It's a landscape game, but works both ways round, and flips sides properly. It has background music, but if you're playing music off the phone already then it'll start up with only sound effects, without asking you anything. It uses touch very nicely, as a direct 'aim here' interaction (backed up with a fine-tuning control) but double tapping anywhere will zoom in on that spot as you'd expect on the iPhone. And it flawlessly resumes from exactly where I last left it every time I start it up, so I'm not worried about leaving a half-finished game when the train gets to my stop.
Also, unicorns!