I've used Ubuntu. I've used Kubuntu. I hated both. I'm a happy SUSE user now, though. SUSE may be a larger distro, but sometimes, if you have the space, the bigger the better.
I've never used Slackware, but I've heard people swear by it. That the next one I'm going to install, but I'm just going to say: Distros like Freespire and Xandros are better for the new Linux user. The next easiest, I'd say, is PCLinuxOS, from there SUSE, then Fedora Core, and then more...
If someone is looking for a distro that is good for all-around use, use SUSE (and, hey; it's free). Ubuntu isn't good for the advanced user, that's for sure. It doesn't even come with gcc, and it's not available in Ubuntu's tiny download repository.