I'm a fan of XBMC. Interface is pretty straightforward, a lot of features, and a lot of plugins available.
I've tried a lot of stuff... Mythbuntu included...
However, I always come back to XBMC, more notably... Openelec (openelec.tv for more information).
One caveat to openelec is that it boots directly into XBMC so if you need a web browser then... your better off going with somewhat something like XBMCbuntu (The best of both worlds)
Hope this helps.