Here are my recommendations:
If you want to learn the ins and outs of EVERYTHING, try Slack. Very few precompiled packages, but very sane filesystem layout. Absolutley no handholding here after install. In my opinion the best overall distribution, it just seems to make sense more than any of the others.
If you want to learn how to use Linux, but don't want to deal with compiling your software and such, Debian is the way to go. Precompiled packages for nearly EVERYTHING, but no handholding.
If you just want to use the OS, and don't want to deal with the details, Mandrake is the distribution for you. Easy install process, decent filesystem layout, can use RedHat packages (important if you don't like compiling

), very easy to use.
Personally, I dislike RedHat (I call it RootHat), so I won't comment on it.
TurboLinux was interesting, but I never really played with it.
Caldera is pretty worthless, don't waste your time.
Personally, I say go with Slack, and if Slack doesn't do it for you, try Mandrake.