My favority OS is probably Ubuntu, just because it was so easy to install and extremely easy to use, with loads of updates - that I trusted.
However, the one I use the most is Windows XP, just because it is what I've been using for the past who-knows-how-long.
I plan on using Ubuntu more, I just need to slowly get accustomed to Ubuntu, Linux, etc.
What has helped me greatly is the array of open source cross-platform applications that maintain a sense of familiarity, such as the Mozilla family, OpenOffice, Inkscape, XAMPP, etc., along with cygwin, which helped me get accustomed to a *nix-styled structure.
NOTE: I think Windows is a little over-represented in that list, seeing as none of specific Linux distro's are included