What flavor of *NIX would be best for a relative newbie?

Mac OS X. *fans flames* :)

But seriously...I'll second that "stay away from Debian" suggestion. Even on a fairly vanilla box that one had me screaming and wanting to throw it out the window, and never could get X11 running. I used to run and was sufficiently pleased with RedHat, but that was many, many years ago...dunno how recent versions hold up against other distributions out there, but might still be worth a look.