This is all personal taste, isn't it? Unless you're talking about facts like album sales, charts or whatever, then all the breaking down of stuff is just - like beauty - 'in the ear of the listener'.
To me, Eminem isn't the best, though I'm not sure who I think is. When I used to listen to rap, I can't say I had a favourite but enjoyed a lot of songs and some of the rappers. Can't remember most of them but in no particular order:
MC Lyte (Cold rock a party remix, Keep on keep keeping on), Da Brat, Foxy Brown, The lady of rage, Trina, Rah digga,
Bone thugs n Harmony (Crossroads),
Tupac (A lot of songs, can't name them all but 'Dear Mama' and 'I ain't mad atcha' are quite touching).
Snoop Dogg, Naughty by nature, Salt n Pepa, Missy Elliot,
Dr Dre (California love...with tupac), Warren G, Master P, Easy E, Biggie (Notorious BIG, Big Poppa), Nas, Nate Dogg,
Eminem (cleaning out my closet album)
Eve, DMX, Ja rule, Big pun, Fat Joe, Jay Z,
Puff daddy (Now P.Diddy), Mase, Loon, Camron (all "softcore" rappers but enjoyed most of their songs).