For major holidays the "general rule" is always that Epcot is the least crowded (or at least has the most land to handle the crowds) and the Magic Kingdom is the most crowded. Often the Magic Kingdom is the least crowded on the day after the holiday (since everyone else visited the day before).
Just to be sure, I also checked the "crowd calendar" predictions on
www.touringplans.com (yes, I have their permission to post their data here). According to that site, the best parks to visit on Thanksgiving are Epcot and Animal Kingdom, and the Magic Kingdom is the most crowded. On the next day they predict that the best park to visit is the Animal Kingdom, and the most crowded are Epcot and Hollywood Studios. Their opinion is that the second least crowded park to visit on the day after Thanksgiving is the Magic Kingdom. So, my "general rule" seems to hold fairly well.
Please keep in mind, though, that "least crowded" and "not crowded" are very very different descriptions.
All parks on Thanksgiving day WILL be crowded! On a crowd level scale of one to ten (ten being most crowded - like Christmas day), Thanksgiving is rated overall as a nine by touringplans.com. Again,
ALL parks will be crowded.
Hope that helps.