There is no "best" version of Linux... It depends on the job at hand.
Ubuntu is the most POPULAR Linux distro at the moment... It is a very good distro for users transitioning from windows, it has good support, certain versions (that are labeled LTS) have "Long Term Support" Meaning you won't "have" to upgrade for many years to receive security updates, etc...
I happen to be booted into Kubuntu at the moment, but on my laptop I also have BackTrack 3 Linux, and Gentoo Linux. Each has it particular uses. Gentoo if I need to set up a temporary router, want updated packages etc. BT3 for security analysis/hacking... I really only use Kubuntu to keep "up to date" with it since I have it on my wife and daughter's PC's, and generally recommend it to friends who are interesting in trying Linux.
I prefer Gentoo, which is also what runs on my servers at work, all my customer's servers, and my own server at home, but I'd be an idiot to recommend Gentoo to a complete Linux neophyte... It has a LOT of advantages over other distros, but isn't what I'd consider a "beginners" Linux.