Debian Stable/Testing/Unstable - hands down, without a doubt.

I have used Mandriva, openSUSE, Fedora and Ubuntu in the past and nothing I've tried so far comes close to Debian. Stable is rock solid and even testing is in a far more stable state than Ubuntu 9.10.

I run Stable at the moment as my main system and if you want something that "just works" and doesn't give you any grief, then Debian is your best bet.