BEST ?
based on what
okay for me
If I want to make a good looking user interface with some beautiful looking gadgets and much more user friendly I'll recommend you a Windows 7 (32 bit)
If you have interest in bit of video editing and some photoshopping I'd recommend Windows 7 64 bit
now if your a Gaming freak, don't have too much RAM and you got a good graphics card, I would ask you to get a Windows XP and you can be a hardcore gamer
i have 2 computers at home one running windows XP SP3 and other has windows 7 64 bit
I like my xp one