I think it also depends on how much Unix knowledge you have. If you are new to Unix then Mandrake and/or Redhat is the one to try first, if you are comfortable with Unix, then prolly Suse, Debian or Slack are the other distros to try.
It is really also a matter of personal choice, you should pick one that you are most comfortable with.
This may sound crazy, but I tell folks to buy or borrow an old hard drive and use it in place of your existing hard drive over a weekend or two. Then install and try out 2-3 distros and pick one you seemed interested in. This way, if you have a working windows (or whatever) config, you won't destroy it and also get the opportunity to check out a few distros. I have only tried Redhat and it is simple to use from my perspective. Of course, the assumption is that you have a high-speed internet connection to try all this, some of these distros can take a while to download.