As far as I've checked/know Basecamp is the big daddy of this quartet, it has oodles of functionality without getting in the way but too expensive for a lonely developer (which is understandable because they aim for groups not individuals). Asana looks like Basecamp and... well I am an avid fan of Kanban boards so this top to bottom todo list approach does not work for me. Do is, I believe, an iphone app and invite only so I do not know about them.
So far it's been awesome. It serves as a private kanban for project management. We also tried it as a public feature request board for end-users, but later replaced it with a UserVoice forum instead, since end-users had to register a Trello account to comment or vote for features, which is a nuisance.