My fav distro really depends on what I am trying to acheive. My firewall box and any Linux server I set up simply HAS to be Slackware. I previously used BSD and switched to Slackware for compatibility with some software I needed to run. I love the amount of control Slackware gives in terms of configuration (while staying within the "rules" of the distro) and it is very close in setup to a UNIX.

On desktops however I love the APT method of package management. I started using Debian with potato, and switched to Ubuntu with the release of Hoary.

Ubuntu is my top pick for a desktop Linux distro.