I too perfer slackware
Redhat in general still focuses more on the corporate desktop and server market.
Mandrake's targets the home & corporate desktop market.
Personally I tend to recomend Mandrake to most people first getting into linux. Though it's not my personal preference, I think they do a good job with their distro.
I'm not totally against redhat, but I perfer Mandrake over them.
Slackware is my overall favorite, because it's kept simple. Packages are carefuly selected, it doesn't have the bloat of most major distributions. It doesn't start near as many services by default. And it is not a RPM based distribution, I got really sick of the whole rpm thing. I like doing things by hand, and in linux you have a lot more control that way.
For more refer to http://www.distrowatch.com/