I like Vista best, once the settings are tweaked its far better.
http://mostlysavingmoney.com/top-10-windows-vista-speed-tweaks/
I have no problems with Vista (had Vista Home Premium for a year) and find all the extra features speed up everyday usage.
e.g.
snipping tool
e-mail button (one click button to copy photos onto a new e-mail and shrink them to a selected size)
readyboost
windows mail
tap preview
quicker to navigate explorer
Etc, Etc, loads of big advantages over xp.
The only disadvantage is Vista takes a little longer to boot up and shutdown.
Vista is very stable, I had more problems with XP than I have had with Vista (same computer as I upgraded xp to Vista).
If I had a low spec. computer, then I would stick with the lighter XP O/S.