I've tried them all and this one is the best, if you like a straightforeward approach to creating your lists. Whether or not you use GTD, AT is handy. For an even more stripped down approach, try iProscrastinate.

There are just 2 features that I absolutely need on AT:

1. The ability to nest projects. That is have Goals or Projects at the top level and Subprojects nested within. Or, even allow multiple layers of nesting, as iGTD does. Right now I use letters to group projects. Thus, "A" for art, "F" for financial, etc as prefixes to the projects.

2. The ability to selectively sync actions to iCal. Using pause is one solution but not the most elegant. On the other hand, I can always narrow my focus with the options in the amazing Agendus Pro.

[BTW, iGTD has a lot more features, including the above two but is just too "busy" for my tastes.]