Ubuntu because of the general audience suitable support Canonical provides, e.g. AskUbuntu on StackExchange. It's where I've settled and though I use Xmonad, I found Unity an improvement on the older WIMP paradigms...at least once I learned to use it.
I'll add that part of the reason for focusing on Ubuntu is that I think it would be hard for a company to build and maintain a critical mass behind most other distributions...and that sort of stability matters to me.
Good luck.