Namely, his 3 solo albums - Return Of The Boom-Bap, KRS-One, and I Got Next. His voice on those is fucking excellent, great work with the delivery (emotions and energy), and he also has great rhythmic patterns. Like, reggae influenced and shit, which leads to a great combination of melody and passion.
This is evident in his song "A Friend" - http://www.youtube.com/watch?v=s_tsaRsEOuU
In addition, the 3 albums have great production, a lot of classic Premier work to be exact. Songs like Step Into A World and MC's Act Like They Don't Know are real alternative but you could also play them in a "mainstream" club and get great response.
