Vista =P
Vista is really pretty, but other than that it is pretty pathetic.
I'd have chosen XP, but my mom claimed that I couldn't get a laptop with it anymore (which I knew was a load, but I didn't have time to fuss around with it).
All in all, it isn't too bad, but I also have a widescreen display, which makes everything other than viewing movies difficult (I can't find wallpapers, games look stretched horizontally, etc.)