I've used most of these apps, but what I am looking for are apps for the iPad and iPhone and a Mac desktop version, plus OTA sync.
Things, Omnifocus, and Firetask have each. You simply can't go wrong with any of these three apps. They all have what I want. They are all equally powerful and easy to use (with the exception of Omnifocus) and have OTA sync. Granted, Things' OTA sync is still in a beta stage and doesn't work very well, these are all three fantastic apps.
I have all 3 of these and have used all 3 of these and found that for my needs, Firetask is the way to go. It's as easy and beautiful and powerful as Things, plus the OTA sync works. If you don't really need that function, then Things would be a great alternative.
Omnifocus is built for the more powerful user, the user who needs the ability to be a project manager on a new aircraft carrier at the Northrup Grumman shipyard in Newport News. That's just not me.
I tried the others listed in the thread and didn't like them at all. I can safely recommend Firetask to the average joe user. I give it 5 out of 5 stars.