Windows 7 pro 64-bit on an AMD Phenom II 3Ghz (quad core) system with 4 gigs of ram. I'm not exactly a Windows expert, but that version of Windows is the only one I recommend based upon my experience with older operating systems and hearsay regarding the newest one.
It was better than average when new but is a little dated now. It doesn't have the raw power of newer systems for cpu intensive tasks but for multi-tasking, photo-editing, 3D modeling, browsing and other basic tasks it responds instantly. So long as I don't install a bunch of rarely used applications such as Skype, Dropbox, Adobe products and over-intrusive MacAfee clones, this computer will keep up with my working pace without bottle-necking me.
Edit: I should clarify that I've not actually owned my PC for 6 years. The tech is 6 years old since I always buy the previous year's technology for maximum cost vs benefit.