asking this kind of question is like asking what kind of pie goes best with ice cream. you're going to get a different answer from just about everyone.
Ubuntu seems to be the favorite flavor of linux lately. I'd say Fedora is probably next. You've also got OpenSuse, Redhat, Gentoo, Slackware, etc. Ubuntu and Fedora OpenSuse) are probably the easiest distributions to install/maintain. I don't think I've had to compile anything from source on any of my installs, except for the ocassional smal/unknown lib or app.
Basically it comes down to this: use what you feel most comfortable with. If it's Ubuntu, then go with that, but there is no BEST linux distribution for Rails.
