Everybody is going to have their own opinion on this. My suggestion? Go for one that is well established and has been around for a while. It's part of the reason I use Debian, it's one of the "original" Linux distributions along with Redhat and Slackware. Most other Linux distributions that are out today were forked from those 3 projects.
If you want to use Ubuntu, I suggest using an older version. In my opinion the newest one (11.04) is crap. It's slow to boot and the default UI is a Beta and unreliable. Even when you switch to Gnome it seems much slower than previous releases.
I've had outstanding stability and performance out of Debian stable, although it's not quite as "user friendly" out of the box as say Ubuntu or Linux Mint.
If you just want a user friendly OS to throw on a thumb drive and surf the web or goof around on while you're out and about using different computers. I suggest Linux Mint or Debian.
Attached is a graphic somebody gave me once demonstrating the history of Linux distributions.
Edit: Apparently the attachment is too big, it got scaled down, here's the original.