Interesting question. I guess it depends on how much you want to spend, and what you're looking for! Poor as I am, I want a good quality souvenir for as little as possible, so I'd have to go with Cedar Point. You can't beat the full-color poster/maps for $1, and they have the best selection of postcards I've seen at any park. Disney has an incredible selection too, I'll admit, and so does IOA, but it all depends on what you're interested in. I like pictures and that sort of thing, so it's CP for me!
-------------
Po!nt of View: A different look at Roller Coasters.
http://www.crosswinds.net/~justmayntz/thrills/index.html