I will start.
I first used Win Xp Pro.
I will openly admit, I acquired a crack for Vista, and I tried to install Home Premium.
Well, that worked for 30 days, and I got it registered with MS, but I didnt get it activated. Although all my friends who I gave the crack too, it worked no problem. I tried it and formatted 4 times.
So I went back to Win Xp; but home this time.