He's not terrible, but at his absolute best he's very mediocre and often times puts out some of the worst rhymes possible. Take away the background track and a lot of his stuff is very simple; the kind of stuff the guy down the block who dabbles in rhyming comes up with in his spare time.
I've noticed that the people who think he's the best rapper ever tend to skew younger and are generally not exposed to hip hop music as a whole. Kanye himself knows he's not the greatest anything, so why do they think so? Because they buy into his troll antics. He almost didn't make it in the rap game because he obviously sucked big time in his demo tapes and no one liked his boring rap style. His delusion is that he was "too different," which of course doesn't explain Busta, Eminem, Outkast, and other "out there" artists making it big at one point or another.
Except he made that background track.