I'd say Eminem. He goes beyond simple rhyme schemes, which I feel most rappers don't do this days. More often than not rhyming the 5-6 syllables of each line as opposed to the last word. Also, he's got diversity as far as song topics, and a wide array of emotions depicted in his music.
The best rapper of all time in my opinion was biggie but the most true to life rapper was tupac. But in todays rap world its Eminem.
You're kidding me right? His rhyme scheme in Encore was soooo complex it was rediculous for the most part. He's never succumed to the simplicity of rhyming the last word of a sentence. It's always the last 5-7 syllables or so. Some of his shit is obviously pop-ish for radio purposes, but you can't tell me his rhymes got more simple as things went on. That's absurd.
from the ones i know: i looove the roots immortal technique (of course!) yesss i've heard (lol!) of rakim & krs1 kool savas (i assume you've never heard of him...) that old 2pac-biggie thing still has some kind of nostalgic romance to it, i must admit... actually there are many more that just don't cross my lazy mind right now (and method man's voice gets me all...wild....but that's a whole different story...)