I previously used all Redhat / Fedora / CentOS. But now I just use Ubuntu. They seem to put a lot more effort into maintaining software compatibility, painless upgrades that don't hose up your system, and have far more up-to-date packages available. And if something does go wrong, their support lists are very useful. But that's just been my experience - and it is very negatively biased at this point against Fedora, because Fedora 9 was a horrible release.