I have a pretty eclectic taste in music, and I do like rap, but I am not a hardcore fan. I tend to like where rap/rock have merged together, in bands like Linkin Park for example.
Most of the stuff that I do like and/or respect is well represented here, and most of it is old school. I like Public Enemy, NWA, and Snoop for example, but I also own most of Eminem's stuff.
I don't feel qualified to give a list of the best 6 of all time (Chris Rock put together a list for Rolling Stone recently by the way), but I do see a couple of old school groups missing. Run DMC and the Beastie Boys at the very least have been pioneers. If I had to pick one album from each it would be Rasing Hell and Paul's Boutique respectively.