I think to pick a favorite of anything is to show no appreciation for the many facets of an artform. There are many different MCs that are dope in many different ways, whether it be flow, word usage, topic, punchlines, etc. (and come on Eminem, he's like the Howard Stern of Rap). If I were gonna name some of my favorites that are alive, I'd have to say the God himself Rakim, the teacher KRS, MF Doom, Aesop Rock, Copywrite, Ikon the Hologram, Eric & Parish, Tame One, The Grouch, and lots and lots of others... The point is that narrowing your vision into picking a favorite shows that you have no appreciation for variations in different types of skills.