The "best" value resort is whichever one you like best.
Pop Century is newer and shiner, and has some different options in teh food court.
The three All-Star resorts are all identical in ammenities, food courts, and size. The differences between the All-Stars are mainly in the layout of the resort and the theming. The All-Star food courts also have slightly better quality than the food court at Pop Century.
My personal preference is Music. It is the quietest of the Values, especially in the areas farthest from Melody Hall. But it is laid out in a long, straight line, so some of those areas quite a hike from Melody Hall. This doesn;t bother me, since I always have a car and always ask for a room close to the parking lot, but if you are relying on the busses, you will want to get a room closer to Melody Hall.