I'm currently using Vista on my desktop at this time. I am probably going to update to Windows 7 soon though. On my laptop, I am running Windows XP. It's still the best OS out there, in my opinion.
Windows Vista right now. I have a really nice hard drive and processor, so I haven't really experienced the "buggyness" of Vista. I like my OS, but I miss Windows XP. I also have linux/ubuntu installed as a partition. I don't really use it though. It's tough to get used to!
Personaly I have been using Last XP v21 for the longest time. Its a hacked version of windows xp and they took out all the fail stuff and replaced it with the best stuff lol. Its decent.