I continue to look for one great iPhone game. But there's a a big market now and Apple's Game Center gives us data on sales. Some notable iOS games:
It's hard to compare more generally; iOS games tend to be platform exclusives unlike big PC and console games. Here's sales stats for Xbox 360 games with a whole bunch of 1 million+ games. Even modestly successful AAA games like skate sell 500,000+ copies. Then again they need to, the budgets are well over $10 million.
What I love about the iOS game market is the long tail, all the weird little innovative games. Like Canabalt, a marvelously perfect mobile game. 78,000 sales isn't bad for a small game, particularly at $3 a sale. (Related: I wish Apple had set the floor on prices at $2 or $3.) 100 Rogues is a great example of a gamer's game, intricate and clever and beautifully designed. It deserves more sales, but I'm glad it at least has some. Forget-me-Not is tiny at 1100 sales but it's brand new and has sharp corners, I'm curious to see how it fares.
Caveat: the data above undercounts. It leaves out people who've opted out of Game Center as well as people who haven't played the game since Game Center support was added. Angry Birds claims 100 million downloads, but I'm not sure how much of that is iPhone sales.