i haven't tried slackware.... the distros i've used/attempted to use are: redhat 6, 9, caldera (the version that was out at about the same time as RH6), suse 5 or 6, 8.2, mandrake 7 or 8, 9.1, attempted a debian install and failed miserbly, and now i use gentoo 1.4 stage 1
I was deciding between Gentoo and Suse, but opted to try Gentoo. Since I'm still fighting with getting X working, I haven't had any time to get a feel for it. Let us know what you like best between the two.
I think Suse will be the next one for me on my mission to try all the mainstream distros... :P