This is an inaccurate statement.
The most, no, but he's certainly up there.
I say he is wrong in both aspects. Kanye isn't really that highly rated as a rapper. At all. People like his music, but they prefer it when he is producing for Common than listening to him. So basically, you can't be overrated if noone but yourself rates you that high.
Now he is definately number one on my list of people I would punch just randomly if I ever had to spend 5 minutes with them. He passed that albino looking dude in the black eyed peas last year and has been widening the gap every since.
That's my argument. He overrates himself, but he gets very little attention from anyone else.