J Dilla
Ant (Half of Atmosphere and Rhymesayers in house producer; One of the most under rated)
Timbaland (& Danja)
Madlib
Dr. Dre
Neptunes
The top 3 are tied. I love Dilla and Timbo for their drums. Dilla has some of the most human feeling drum rhythms.. he really thought like a drummer. Timbo, I love the use of live drums and auxiliary percussion. Gives tracks more human feeling too. Ant finds some of the funkiest samples.... Madlib would have to be next (obviously if you read the list
Dr. Dre and the Neptunes I don't follow religiously but I respect their work and love most of their tracks on albums I have.