sorry, I think Chicago O'hare is definitely the worst. My personal theory is that it's part of a program to improve the quality of life in the actual city- they take all the miserable people off of the streets and keep them at the airport. I fly through there at least four times a year, and I've run into a grand total of 2 genuinely friendly people! Come on, even a smile would help!
As far as the best airport, Edinburgh gets that, in my book, for the opposite reason - scots are actually friendly! Even their security people are nice.