Otis Jackson Jr. (far) and James Dewitt Yancey.
Say what you like about my bias towards both these producers, but once objectively analyzed their catalogues are easily enough to qualify them for musical virtuosity. Dilla had perfect pitch, and a sense of rhythm that could surpass the most sophisticated sequencing programs, and his engineers and close friends can attest to the fact that he crafted most of his beats in 15 minutes or less.
Madlib is an entirely different beast altogether; he was self-taught in over 20 musical instruments and proficient enough in their execution that he was able to record a staggeringly large discography of jazz albums, many of which have become heralded as contemporary classics already.