If you like to play games, make sure you have a video card, it is very important.
VISTA!!!
Sorry, but I'll have to say XP
It let's me play Garry's Mod Better :)
XP. However, I am using vista now. The problem with vista is I don't really see a big improvement over XP. I have had a ton of problems with vista trying to get stuff to work that had no problems in XP. I'm talking basic stuff like usb drives. I had to go on to microsoft site to download a driver that should have been included on disk. The problem is it took me about 2 months to figure this out because no one else figure this out. Which reminds me I need to go back and post the fix to my problem to help other people.
Mind you vista does work. I just don't think it is worth all the pain I put up with.
I would suggest if you wanted vista and the game compadability that xp provides. Set up a duel boot system using two hard drives. Place vista on the C drive and XP on the other drive.
-icurnvs