Considering I, like many, didn't expect the mobile phone to really take off as a gaming platform but that said I have quite a few games on my iPhone which I play on a regular basis. My favourites are;
Zenonia 1 & 2
Cut the Rope
DoDonPatchi Revolutions
Espgaluda II
Bejeweled 2+ Blitz
Lets Golf
Sonic 4
The last 2 you can probably skip as I got those for free but everything else is excellent. Oddly enough, I didn't get the hype over Angry Birds. It's the same flash game we've all been playing for years. I remember when it was Penguin Olympics.