Toodledo:
http://www.toodledo.com
There is nothing better. I use it all day long on web and iOS. I've been using TD almost since it launched. I don't even use that many of its features - just folders, some light tagging, and repeating tasks. There's also contexts, goals, length/timer, priorities, stars, locations, etc.
In truth, I have not played with Wunderlist much. There was a big blowup when they announced "well Windows 8 is out and so we're no longer supporting Windows 7" which I thought was monumentally stupid. Not that you really need a desktop client for this sort of app - I just use the web interface for TD. The only reason I have the iOS app is that it makes "unlock phone-hit TD icon-hit add task-enter" really fast for capture.
TD also supports shared task lists.
Here is a comparison:
http://www.technorms.com/33564/wunderlist-vs-any-do-vs-toodledo-vs-gqueues
People also use OneNote or Evernote for GTD setups. In truth, you could use text documents on Dropbox - and in fact, I know a couple old-school guys who do just that.
GQueues looked interesting but it seems to be one guy which makes me nervous considering how much I rely on this tool.
For true group task management/light project management, I like both Asana and Jira. I've heard a lot of people like Trello.