I usually lean towards Debian based distros. I like the .deb package system compared to .rpm. It's a cow to install from the Debian disks compared to the other, more slick distros. Roll on the next release!
I had Corel Linux dual booting on a PII laptop but tried others too. I'm yet to reinstall Linux on to it following a reformat. I can't find a solution to needing to use an external monitor after the screen died.
I have Mandrake 9 on an old Pentium 166. It took damn near 24 hrs to install & runs rather slowly. I haven't HDPARM'd the HD yet. Knoppix didn't seem to like it.
I've been looking for a distro for my new laptop. Knoppix worked rather well. Wasn't game to try the beta Knoppix HD install though. Hadn't backed up my data recently. Just got a copy of Xandros which is also Debian based. Have yet to burn the .ISOs.