I looked over quite a few of the tools listed on the wikipedia page http://en.wikipedia.org/wiki/Comparison_of_GTD_software
, and ThinkingRock really impressed me with the way that it guides you through the GTD process. It was also important to me that I could share the same list between my work and home computers, so the ability to just send the xml file was really useful.
I've only been using ThinkingRock for a few days, but already it's really showing it's value in helping me to Get Things Done.
And the price is right!