well, to answer that would a LOT of time and space.
i guess i base my opinion on comparison, history, and aesthetics. plus, there's that 'magic something' that just clicks with the very best work. it's a sense of total perfection and utter mastery. he has that in spades in my eye.
jamie and i were hanging with him last time i was in nyc. he's going through a few "life changes" right now, but his work has gotten even more 'out there'. he seems to be re-inventing his style. i guess the copycats overwhelmed him.
have you ever seen james victore's poster work? sagmeister took it to 'new' levels all right - LOW levels.
i like stefan, but i think his work is a little too derivative (i can show you where he stole almost everything in his book) for my taste. it's sorta like paula scher - great salesman/copycat designer. you guys call me a theif and a hack, but you have no idea how bad it can get out there when you're a good salesman and privvy to the arcane history of graphic design. you can steal forever and look like a genius.