What's your favorite Linux distribution on the server and the desktop, and why is it?
Arch Linux No frills, no flashy stuff. All the control and flexibility of a source-based distro like Gentoo, without all the hassle.
Bleeding edge repositories, great community.
Centos for stability - tracks Redhat enterprise, no nasty problems for us in 3 years.
CentOS is an Enterprise-class Linux Distribution derived from sources freely provided to the public by a prominent North American Enterprise Linux vendor. CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible.
Debian for me.
It is (so far) the most stable distribution and provides the best (IMHO) package management system.
If you need anything else that is not part of standard Debian, then have a look at the many derived distributions. A noteworthy one (IMHO again) if you prefer to be up to date is Sidux.
Debian and Ubuntu are recent weapons of choice.
That package management system is built like a tank.
For the desktop I would have to say it's the new version 9 of Ubuntu that I am currently using to break my current record of staying away from Windows for the longest amount of time which is currently 1 day :(.
For the server I like to use a clean Debian install and build up from their adding the packages I need for each build.
Arch Linux for upgrade ease - there are no releases, just newer packages that, when stable, are released into the package repository.