[lvlamb]

Geez! Yes... err ... wel ...
Voted for others.

I like Debian, largest ports collection available, but on -current. These are kanopix and sidux.
Also like puppylinux, rather the deviant grafpup. (Read what he says about first user=sudo).
Just depends on the usage: appendable CDROM liveCD or ports availability.
Now, for any other usage, it is Slackware. At least with slackware you don't have to search for *-devel or headers and can GNU-make sources.

Now, for Xen VMs I would choose CentOS (although getting the source collection for a given CentOS kernel in a real pain in the "a"{dollars}.
Better go OpenSolaris.
I used to install Ubuntu, but can't get my TV cards to have sound before breaking an app (have to break kdetv with no sound, to get motv working with sound and kdetv exiting with errors). Farewell Ubuntu, but without me.

Haven't a goot working VMware anymore.

Lastly, some apps only run on the distro they have been written for (typical Linuxism, bash-ism, UTF-8-ism, ...) . Hence, like it or not, I sometimes need one distro for one app.
Thanks gawd, there is GRUB.

All in all, slackware for debugging purposes, grafpup to cd/usb boot from anywhere with my desktop apps. OpenSolaris to keep informed.