Things gets my vote. The syncing gets to be a pain, but I have the Mac, iPhone (ipod touch actually), and iPad versions so I just leave a Mac on. I also have 2 macs synchronized through dropbox (which is also a pain when you get conflicts).

I've tried RTM, TeuxDeux, among others, and they just aren't as amazingly intuitive as Things. There's something about the interface and the way it's set up that just completely gets out of your way so you spend less time tinkering with your task manager, and more time doing the stuff on it.

If they could get cloud sync, it would be perfect