Ubuntu Hardy Heron
I highly recommend Hardy Heron, it fixed several noticeable bugs in Gutsy. No longer does evolution start up with error messages about being unable to connect to my POP email (even though it works fine after it starts up). Accessing FTP and SSH ftp sites seems to be much more stable with the gvfs conversion. At first video playback was very slow, but then I did some searching about, and found the problem was XGL. So if video performance seems worse after the upgrade, try removing XGL, you can run compiz without it now.
The only downside I can find is that Firefox 3 beta 5 (while definitely being better than 2) has some stability problems, which I'm sure will be worked out in the next month or two.