Currently Ubuntu; likely switching to Debian in the near future.

I was a huge Gentoo fan early on, but switched to Fedora when I stopped enjoying the USE configuration (which I still believe is awesome, just way too time-consuming), then to Ubuntu when I stopped enjoying yum (well, I never enjoyed yum). I think aptitude is the most stable package manager I've ever used.

Likely going to Debian soon, though. Trying it out in a VM (love those buggers!) It doesn't matter a whole lot since I run openbox, but as a developer, I think knowing the package manager well makes a big difference in the quality of the experience.