1. Ubuntu has the best out of the box configurations and one of the largest software libraries. It also has an easy to use driver installation service that automatically sets up things like xorg.conf for you so that you don’t have to mess with it or compile the driver from source. apt is an awesome package management software and .deb > .rpm. No package selection at install.
2. Fedora also has a mega repo but most of them are “release early and release often” builds that can sometimes be buggy. Most commonly used software will be stable enough to use. More cutting edge or new software will be more buggy. Uses yum which is a decent package manager and rpm packages which are well supported and easy to understand once you get the naming scheme down. More installation options (package selection) can either make your install easier or more complicated. Generally has a newer kernel than Ubuntu.
3. OpenSUSE only just became “open” a few months ago. It hasn’t followed many of Linux’s “fundamental philosophies”. It does have a nice library and is supported by two mega-corporations (Microsoft and Novell). For these reasons, I’m not a huge fan of “Open”SUSE.
4. Mandriva was the original Ubuntu. The “Linux for dummies” back when it was still Mandrake. For this reason it’s also good out of the box like Ubuntu. If it were me, Ubuntu would be #1 and Mandriva #2. It also uses rpm package management which I’m still not a huge fan of.
You probably knew all of this but maybe some people didn’t. I’d go with Ubuntu (and did) and never look back.