This is sorta like asking drummers who the best drummer in the world is. You're not going to get a convergent answer, and in the process you may just get a bunch of wailing and gnashing of teeth. :-)
I think the best version of Linux -- particularly if you're learning -- I think is the one for which you can find the easiest support. What do close friends/coworkers use who are more experienced than you who wouldn't mind answering a specific question of ya once in a while? Or who you can actually sit with from time to time?
I think for a noob though, it's hard to argue too much with Ubuntu or a hard drive Backtrack 4 (which is Ubuntu based). You'll find loads of online support for either in either the Ubuntu and/or Remote Exploit forums, and it's an easy enough distro to maintain that you're likely to find users of it locally or among your circle of peeps.
There is, however, a case for a noob that has some time and wants to REALLY learn how everything works under the covers, to go through the pain and the agony of installing and keeping Gentoo Linux up to date. I'd recommend this only if you have a fast machine that's going to be hosting it (because it'll be compiling stuff a lot), and you have a patient buddy who runs it that you can bug with questions. You'll learn a ton going the the Gentoo Guide step by step, but it's not pretty. Myself, I'm over it and don't have time for gentoo care/feeding, but I did learn a lot that other more friendly distributions these days keep you from ever seeing/caring about. gentoo.org
Good luck and enjoy!