Some of my favorite older groups were:
PRT
TCQ
Beastie Boys
Run DMC
X-Clan
Eric B and Rakim
Cube
Too Short
NWA
As far as new artists go, you can't deny Em.
By the way, I have never understood the infatuation with Biggie. What, in all seriousness, was it that made him good? I always felt that his tongue was oversized or something because he did not have a clear voice at all, in fact, most of the time I couldnt understand him. I liked some of the songs, but understanding it would certainly help...maybe he had good lyrics??
This may be a stretch as a "rap" group, but Urban Dance Squad would have to get consideration for best group of the 90s.