Personally i reckon people like Snoop Dogg, Dre, Eminem and Nas are all crap now. They used to be when there was not really any money in rap and their songs where actually slightly meaningful. Now they just make songs that appeal to the masses - songs about money, cars and hoes.
Each good rapper is good for different reasons. So i think it is hard to compare them. The only way would be, by record sales, and that is based on the opinion of society and not on the skill of the artist.
Oh yeh, and Nate Dogg is cool.