Ubuntu!

I've been running one kind of unix or another for 10 years now. I liked SuSE better than Red Hat for RPM based distros. Slax is pretty good.
Debian distros are the best IMO. Ubutu is pretty good. Raw Debian is good, too, once you know your way around the OS. I have settled on a distro called Mepis for desktop use. It was based on Debian, then Ubuntu, and now back to Debian again. This is based on where the software repositories are maintained. A great "toolkit" distro is Backtrack.
I keep a Win98 and WinXP partition around, just in case I need to open some weird spreadsheet or Word(r) document, but I don't use them very often.
I like being in control of my OS. I don't trust Windows much for browsing.