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
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.