All the cool Mac users swoon over Omnifocus. Omnifocus sure seems great, although a bit pricey and complicated (neither of which I mind very much). The biggest issue I have with Omnifocus, however, is the fact that it's Mac and iOS only. I try to spend as much time as I can in the Apple universe, but for a big chunk of my day I'm using a PC and Android phone for work.
After trying a few apps I settled on Doit.im. I like the fact that it has apps for all the major mobile platforms and it has a nice web app for me to use in the office. From a GTD perspective is has all the things I need. I can quickly capture tasks, assign them to contexts, group them into projects, and create reminders and alerts. I can even create tasks by forwarding important emails to Doit.im.
There are definitely fuller-featured apps out there, but this one is scratching my itch.