Right now I would say: None of the above
I installed Ultimate (via MSDN) to test it on my home desktop on Friday night. I put XP back on last night. Why?
1. The Audigy2 drivers (only 'beta' available) suck. To get a reasonable volume level I had to set max volume on my system (in Vista) and 3/4 volume on my speakers.
2. Firefox installed and worked until I closed it. After that any re-launches resulted in an error - something about a previous installation already running (according to Task Manager there wasn't)
3. Ghost Recon: Advance Warfighter. Installed fine. Launched multiplayer and it wouldn't take my GameSpy account (verified working web based). Patched GR:AW and after that the game wouldn't launch anymore.
4. The default 'Explorer' file browser theme/view is awful. I don't know if there's a way to use Aero and get a 'XP style' Explorer view or not. I didn't have Vista installed long enough to try it.
5. UAC. I know it's there for a reason (security) but it's just too annoying. If I ever install Vista again I will disable it.