Damn. I'd love to say "Biggie/Tupac" but fact is rap is evolving all the time...over the years rappers have gotten better. Its unavoidable. I dunno, I have to take into account context, how influential the person was in their era, how they compare to their contemporaries and other stuff...
I'll say this now yh. THIS IS THE BEST RAPPER, NOT THE BEST ARTIST. When I'm judging, sales are kinda irrelevant.
Can't pin it down on one person, but stand out acts (for me and in no order):
Big L (its like people forget about this guy)
Lupe Fiasco (pre-Lasers)
People are going to "internet rage" at me because I like Eminem and say he is "mainstream", etc, but their problem is, is that they can't handle another persons opinion because its different to their own.
I mean I hate Lil' Wayne with a passion, but If someone said they liked his music I wouldn't have a problem with it, unless they started preaching to me or trying to make me listen to his music.