Maybe this helps for finding good distribution that will suite your needs. As I worked with all of the above mentioned and also with RedHat/Centos.
If you need a distro that just does what it should go with slackware or debian. For sure slackware has some learning curve but afterwards you are able to administrate nearly every *nix like system.
I'm not using slackware anymore so can't say about the package managment but I'll guess it can compete with the one of debian.
Debian is a bit more tweaked *nix than slack and might be a bit easier to adopt. As I'm working with debian right now I'd choose it. Specialy cause there is a nice feature of automated installation of clients and servers. http://wiki.debian.org/FAI
. Also it's a one shot to get a central repository so clients can be easily updated.
When it comes to SuSE I can only say that it got as much to do with *nix as a Tree has with the sea. In my opionion its made for the Windows used mouse clickers. And I did not had a good time with this lady. It just sometimes refuses to do what I thought it would.
RedHat costs money so might not be an option for you. Rather go with CentOS which is a free redhat derivate.
Both are nice and clean distributions.
Maybe you should install all of them on a machine and see what you like and have a flavor for.
All of those come with a more or less GUI installer.