August 5, 2013
Hard to say because I haven't invested in an iOS device yet, Android user here. But I love Flipboard and Springpad, both of which are on iOS and Android.
This isn't really iOS SDK specific but the use of Objective-C for both favorite and least favorite. It was my least favorite at first, but after getting used to it I find it to be quite a nice language.
Theatre actor turned iOS/Android developer and lovin' it.