I tried Fedora Core as my 1st distro, it was a bit heavy. My next outing was with debian, which was nice, but I quickly killed the OS by installing a lot of nonfree software out of other distro's deb packages, and fighting with metapackages. Have been using Xubuntu since then, and very reliably.
I kind of want to try Fedora again now, I use Red Hat Enterprise Linux a lot at work, and find it all to be very correct, very solid. I use SuSE Linux Enterprise Server a lot too, and it's not nice, everything just seems to be a hacky patch, like bashing Linux with a rock until it does what they want it to. There's the odd Debian machine too, and they're very reliable when you're not trying to hack nonfree stuff onto them.
Yet more OSs, there's a lot of AIX, which has a rather nifty set of built in commands to query or set anything quickly, unfortunately I know almost none of these commands, so it's always a fight. It uses ksh with vi keys too, which is OK when you get used to it, old school. Only trouble is it's quite hard to find anything to use them for, the desktop versions usually become hugely powerful, really expensive terminal emulators. If your using your own software with DB2 they're generally pretty hefty workhorses, especially the regattas, but then you need to learn to drive a HMC, it's all very enterprise.