The diversity of opinion on this thread makes it clear that this truly has a lot to do with personal preference.
Ignoring FFD: I find the food and exhibition station in LAX to be excellent, with MIA tied. I have always found the JFK buffet to be sad, but the Bridge menu options make up for it, and then some. ORD usually has nice food, but I have never been there when the exhibition station is open (does it ever open?). I find the DFW buffet to be the worst of all of the clubs.
JFK can be very crowded in the main area, but the mini-booths are nice, and the Bridge is usually nice and quiet, if you're looking for that. I like the seating options at LAX and MIA a lot. The option for places to eat at DFW are poor IMHO, but the places to sit to relax/work are great. I would prob say ORD has the best collection of seating options -- nice calm places to sit and eat and areas with lots of different levels of activity to sit and work. I don't like that LAX and ORD don't have computers in the FL (and I'm not sure about JFK, but I don't think they do, but I could be mistaken).
For FFD: I had great experiences in LAX, with excellent food was great and service . Same at my one time at JFK. At my one time at DFW, service was very unpolished (but polite and friendly and she "tried hard") but if you really care about service, this is not for you. However, I had my best FFD meal at DFW (the ribeye was excellent, even by "high end steakhouse" standards, and it was nicely sized, and the fried oysters entree was on point...and appetizers were also good). In MIA I have generally had wonderful service and food experiences, but I had an experience with very poor service and only OK food.
MIA is the best if you're looking for newspapers to grab for your flight
