I would pick C. Flying Fish seems to be the less stressful choice after which will no doubt be a very crowded park day.
1. At FF, a server will bring you your dinner so you don't have to deal with the crowds at the buffet.
2. You won't have to get up and down out of your seat a billion times to help the kids get their food, which never seems to coordinate with your plate being empty or anyone else's at the table for that matter!
3. A quieter setting like FF is going to be more relaxing after all of the stimulation of constant entertainment all day long.
4. Because there isn't any entertainment or any characters at the meal, you would have time to enjoy your family and talk about your day.
5. Renting a surrey bike or strolling the Boardwalk could be a lovely end to your day.
I haven't been to FF yet, but I've been to the other 2. I like them very much, but I am guessing that the food quality at FF is superior.
Having said all that, I should tell you that I will be at WDW for Thanksgiving this year, too. We are going to...the Hollywood Brown Derby for the Fantasmic Package. I know, I know. I'm sure it is going to be wall-to-wall people at DHS that day, but it is a personal mission of mine to see Fantasmic after many failed attempts.
We were at WDW for Thanksgiving 2 years ago and had dinner at Teppan Edo and had a blast. I'm sure you'll have a great time no matter what you choose.