I have XP, used Vista for a little after it came out but it's just not worth having. Far too pointless to use Vista over XP when it uses more graphics than an OS should. Personally I think Linux is the most efficient although I don't use it.
Umm i got all versions of windows/dos.. from win3.11 untill longhorn & vista tiny (stripped and fast) , all server versions and 2 datacenter versions
+ solaris
I use win2k advanced server because its fast and reliable