I dunno you need to introduce more criteria for that I think.  What era? How prolific were they?  Underground or hit producers?  I think you need to consider people like Grand Puba, Big L, AZ, NAS, Biggy? Its a subjective arguement but there are too many amazing emcees past and present to even begin to shoot for a top three.  I think a top 20 would be more doable, God damn I love hip hop!