I'm not sure if this topic will survive the no flamewars rule. But in case it does, my choice is Arch Linux. I like it because it is lightweight and fast. The base system is very small and you add to it just what you want. Archlinux with XFCE desktop on my old PII-350mhz with 256MB ram was faster than my Athlon XP1600+ with 3/4 gig of ram running Win XP.

It isn't designed for novices, but if you are willing to learn it will teach you a lot about linux. The system is installed and configured at the command line instead of using GUI tools and helpers. And the community/forums are pretty knowlegeable and helpful.

edit: Fix typo at end of 1st paragraph.