Here are my two lists:
Good Enough:
Things (mac + iphone)
Omnifocus (mac + iphone)
Toodledo (web+iphone)
Remember the Milk (web+iphone)
I find that I like to have actions associated with projects, but do better not planning too far ahead, as in canonical GTD. I'm currently using Things, which is ok. I wish the iphone app supported focus areas, but I make do. Omnifocus has a rather slow sync, is a bit like driving a truck, but is good. Toodledo is the most flexible of the web apps. RTM has a very nice iphone app, but I don't care for the web UI- it's too easy for me to do the wrong thing, and it's not all that flexible. It would be good for someone who wants to do extensive tagging, though, and it's completely adequate for orthodox gtd.
Not Good Enough (for me- YMMV):
Nozbe (web + iphone)
Pocket Informant (iphone; syncs to Toodledo and Gcal)
Nozbe never made enough sense to me for me to get all my projects in- completely unappealing and unintuitive UI. RTM is most often mentioned as the best web app, with Toodledo second. I haven't seen many reviews that favor Nozbe. Pocket Informant has so far been a real disappointment. It might be good for those wanting a gcal-syncing calendar, but it's no good to me for my lists. Equating todo tags with calendars makes it hard to use the web version, because tags don't autocomplete there. Using folders for projects doesn't work for me. Filtering is primitive. Although labeled as GTD-supporting, it seems geared for people with relatively few "projects" and long lists of actions for each project, with the top one being the next action. Really opposite of how I work: lots of projects, one to a few next actions per project. There's no good manual, and many of the UI choices related to color seem poor.