I first tried cocos2d, probably the most popular 2d game framework for iOS. I built two prototypes with it... but struggled to get either app to a state where I was happy with shipping.
Then, I happened across Sparrow. I realized its architecture was very similar to ActionScript 3. In my previous life, I had built a lot of stuff in Flash. So, I was able to build apps much faster once I found Sparrow.
Recently, I've been keeping my eye on MOAI, which allows you to publish games to both Android and iOS. I might built future games with MOAI.