Windows Vista because it came with my laptop and I have zero inclination to buy anything different and I don't find it to be a problem.
Windows 7, by far the best they've ever made. I would prefer some variant of linux but I've yet to find one up to par.
Ubuntu linux. I like its ease of use and lack of malware. The only thing its missing is PS (and NO, gimp is not a replacement).
Fedora 11 buh currently using winXP from MSDN coz i have my domain account passwords stored in XP's browser.... Otherwise, Linux all the way!