I'm also looking for something similar. My only real requirement is sub-tasks though.
Got To Do seems nice, but to get sub tasks you have to pay for a pro license for ToodleDo, after you pay for the app itself. I'm sure its not expensive, but still...
Action Complete doesn't seem to have sub tasks.
I've also checked out Astrid and OpenLoops.
I thought the whole idea of GTD was to break a task down into smaller sub-tasks so you can "get things done".
I think I was actually using ToodleDo on my iPhone, but I don't recall having to pay to get sub tasks. Perhaps that is new.