I really think that Eminem is, in my opinion, the second greatest rapper of all time, the first being Nas who I don't think you know of.
Either way they both are what you call "Real MC's", these guys can rhyme and tell a great story at the same time.
As for Metal and Hip-Hop collaborations, you my friend couldn't be more correct.
From Ice Cube and Korn to Anthrax and Public Enemy, those are just few of many great examples of music period at it's finest. Being that Metal and Hip-Hop are the minorities of music, I think when they combine together they really balance out each others differences (musically) and complement one another. It's hard for anyone who doesn't listen to Rap & Metal not to judge the genres right away because they are so different from what "Mainstream" America is used to. Thats quite alright though, because it's better to be different than the absolute same, so who cares!
By the way, would you say that Fishbone and Living Colour are Metal?
I listen to some Metal music like Metallica, Black Sabbath, Judas Priest, And Iron Maiden, but I never knew if Fishbone or Living Colour was more Metal music than Rock.