Personally I'm used to RedHat/Fedora distros but I know there should be better choices in terms of speed, use of limited RAM, etc. Let's assume that disk space is not an issue.
I've also tried Knoppix which I find very nice and yet I haven't got time to check Gentoo. But what about Mandrake, Cobalt, SuSe, and others?

Comments are very welcome.

Regards,
Cacarulo