For a big place I find Chicago O'Hare (ORD) to be very good. It is massive, but really easy to get round because of the little trundle train. Mostly well signed, well staffed and having lots spent on it to bring it bang up to date.
Sorry to say my home airport of MAN is becoming shambolic, more like BAA everyday. More money on shops, less and less on actual airline/aircraft infrastructure.