Which One?
Whatever Linux they use. Seriously, you're only going to get a series of favorites with a question like that… you have to be more specific in what you're looking for from a Linux.
That being said, I am personally a huge fan of Gentoo, although I use Debian a lot, too. Both are solid and have fantastic package managers.
Here are some threads that have addressed this same type of question, only from more specific viewpoints:
http://www.hellboundhackers.org/forum/viewthread.php?forum_id=16&thread_id=8827 http://www.hellboundhackers.org/forum/viewthread.php?forum_id=16&thread_id=8386 http://www.hellboundhackers.org/forum/viewthread.php?forum_id=16&thread_id=8308 http://www.hellboundhackers.org/forum/viewthread.php?forum_id=16&thread_id=7527 http://www.hellboundhackers.org/forum/viewthread.php?forum_id=16&thread_id=7355
Those should help as far as opinions are concerned.