I’ve played with a few, and my favorite is Remember the Milk. Things was nice, but I didn’t like the syncing, and the lack of sharing.

OmniFocus? It allows you to create to-do lists by context and also contained in an organised folder structure. Any task can be made to recur in any combination possible.

A full featured desktop program syncs with the iPhone app seamlessly, each designed to suit the platform. The one problem is that it can be a little complex to new users and is one of the more expensive ones of the bunch. Still if you’re a GTD fan, I’d def recommend it.

Not a precise answer to your question, but as a GTD’er, I’ll share that my main capture app is iTalk. I find it better than typing a note manually – just saying it, and then listening to it when I get to my desk.