It's hard, really hard to choose. Ignoring lyrics, and focussing purely on producing (Kanye actually has a flow, Dre gets his protegés to write his), maybe we can compare achievements.
Kanye introduced the speeded up sample thing, and has consistently produced bangers for himself and anyone who can afford him. You can't ignore that, and I might even go so far as to say that I prefer his stuff.
But Dre created G-Funk, and that sub-genre ran hip-hop for years in the 90s, and was arguably what gave it its crossover moment. White kids loved it, for the music and the violent lyrics, and The Chronic and Doggystyle defined that sound (to an infuriating degree. All the lesser-skilled ripoffs of Dre's production in those years were the bane of my musical life).
And that's not getting into his NWA work, 2001 or kicking off Eminem and 50 Cent's careers.
Got to give it to Dre if I'm being objective.