I've been using Eclipse/StatET. However, there's a new FLOSS IDE that was released today that looks interesting called RStudio. It even supports Sweave, so once it becomes more polished I may switch. User input seemed a little sluggish when I played with it earlier today however.
They have been actively updating RStudio every month or two. It's been months since I encountered even a minor bug. And the features added are really useful--seamless package development and native Git support being the two that come to mind.
This is ultimately a very subjective question, and will depend on your OS, etc.
I prefer Eclipse (StatET) because I use it for all my development and it has many rich features (e.g. SVN integration, Sweave/Latex integration, project management). If you use Emacs, then ESS is great. This clearly depends on what you like.
I used Tinn-R before, but ultimately I prefer to have one IDE for everything, and Eclipse has a much wider general adoption.