Not that I want to bump an old thread, but I thought I should give my opinion:
There are literally hundreds of Linux distros. The best way to pick one is to try a lot of them. If you want a fast answer, though, I'd say RedHat if you want to get a paid job in the future as a Linux admin. RedHat is pretty much the standard nowadays.
You can find much more info about distros at www.distrowatch.com
Sorry again if it's against the rules to bump old threads...