Which Distro?

The first question every linux user asks is "Which distro do I use?". This topic usually generates much debate with everyone recommending their favorite distro.
I normally try to be distro agnostic when discussing Linux, so how do I handle this question? Well, I would first ask them "What's the Linux box gonna do for ya?"

Here are my recommendations (In order of personal preference):
Desktop - Mandriva / Ubuntu / OpenSUSE / PCLinuxOS
Server - Fedora Core / OpenSUSE
Old PC (PII, 128MB) - Zenwalk / Vectorlinux / Slackware
Very Old PC (486, 32MB) - DSL(Damn Small Linux) / DELi / Puppy

To set it straight, I have a soft spot for Mandriva (Formally known as Mandrake). For no particular reason other than I'm comfortable & very familiar with it, I've been running it since 1998 & it makes a damn good desktop in my opinion.
I've also tried out Slackware, Fedora Core, Debian, Gentoo & many derivatives like Ubuntu, SimplyMepis, Zenwalk & PCLinuxOS.
After evaluating all the choices I settled myself on Mandriva & Ubuntu for modern desktops, while Zenwalk & Vectorlinux helps me breath life into my aging PCs.

Of course everything above is my own personal opinion. If you won't take my word for it, here's a website where you can answer some questions & it will recommend the most suitable distro for you.
Linux Distribution Chooser