Like someone else said it all depends on what you want to do. I voted for RedHat/Fedora since I am mainly using Fedora Core 6 with Parallels (http://www.parallels.com) on my XP laptop and MacBook black. I also use and like Ubuntu (which is Debian underneath) very much. I also recently installed Debian in it's non-Ubuntized state. All these I have running under Parallels, which still has a lot of work ahead of them, but it's nice to be able to use the same virtual hard drive file on a Mac or a PC.

Damn Small Linux sure is a fun distro. Weighing in at just around 50MB. http://www.damnsmalllinux.org/

I got my feet wet with Yellow Dog Linux for PowerPC. I think they put that dog to sleep though.