Debian. Been running debian stable on 99% of my servers at work. And debian testing on the desktop, and daily driver. What orginally made me switch from redhat 7 was how frequent i ran into rpm hell, and how difficult it was to do an inplace upgrade. When i could just dist-upgrad to debian woody and everything worked, with a few well documented tweaks, I was sold. And have been running Debian on everything since 2002 ish.
It is stable, reliable, and dependable for the most critical applications. Truly the universal operating system for me.
Edit: forgot to mention that on the 3 desktop machines i prefer KDE. It looks and acts most similar to amiga os, that i grew up with.