Answering in the spirit the question intended I have to say JSON Framework because I use JSON quite a bit and it does what it says it does.
Although I still tend to do more stuff in C than in Objective-C.
I think the spirit of the question was Objective-C frameworks. Also, I'm not 100% sure JSON counts as a framework, jQuery maybe, but not just JSON alone.
I've never used them directly, but I have looked at the OmniFoundation stuff before.
No ****? JSON Framework is an Objective-C framework called, guess what? JSON Framework. Clever huh?