Rules! is a good representation of what the best iOS games are; clearly made for the medium, simple core mechanics and controls that fit perfectly with the platform and no extra charges for dubious artificial gameplay boosts. Of course, the quick, almost minigame genre isn't the only thing you can do on iOS devices, but Rules! pulls it off quite well and is an interesting little game.
The basic concept of the game is simple, you have a rule about how to tap on a 4x4 grid of pictures with numbers, then when you're done it adds a new rule. You go on and on for a while - and you lose if you run out of time - and after a bit it adds new pictures and the rules get more complex. It's a fine little game of memory and speed where there is more challenge into remembering a string of five rules - tap adjacent numbers then tap birds then tap nines then tap red creatures then tap everything in descending order - than the actual act of taping on these big squares.
And that's what there is with rules, you can play as many times as you want, trying to beat your high score, but if you're not into that kind of thing, you might stop after getting really far. That's my only issue with this game - albeit a very small one, when you feel like you're done with it, you are.