I've stayed at ASMusic, and Pop, as well as POR...some multiple times. The best bus service? Pop, hands down. Why? It makes no other stops once it leaves the resort...period. At the AllStars, you may have just one stop, or you may have 2 or 3...your bus may include all three of the resorts, or just yours.
At POR??? There are multiple bus stops throughout the resort. If the driver doesn't see anyone at a bus stop, the bus usually keeps going. But, if you board the bus at that first, main stop, you may have another 3 or so stops to go, before heading out to a park.
So....if I had to rank them?
1. Pop
2. AllStars
3. POR
BUT.....I would stay at any of them again. I found no reason for complaint with any of the buses at those resorts.