Debian or Ubuntu. This is the most widely used base on the desktop. Debian stock, Ubuntu and Linux Mint make up for a huge percentage of Linux Desktop usage. So this simply makes sense. More here

And lay out, piece-by-piece, what I think would be the ultimate Linux Desktop Distribution. And, when I say ultimate. I mean ultimate. The best eye candy. The best media related apps. The best usability. The kind of Linux distro that, out of the box, knocks peoples socks off and makes Windows and MacOS X die-hards lay down their swords and declare this new Linux distro “totally teh rad“.