We always go in late September (
free Dining Plan

) and I've stayed at all the All Stars I think 6 times now.
I pick Music as the best:
1.) I have never failed to get on the first bus except for a couple time returning from MK after fireworks.
2.) If the bus is shared (not too often), you are the middle stop. I've seen people not allowed on at the last stop because of crowding a couple times.
3.) Returning from a park, you can take any of the three All Stars buses. It's an extra 2-3 minute walk (if that) from AS-Sports or AS-Mo back to Music. This has saves us plenty of times when the line for one of the other resorts is non-existant, and Movies is long.
I've never done POP, but I have seen the lines for return from parks, and they are mostly huge compared to the All Stars lines. Maybe they have more buses because of the extra size of the resorts?
All in all, I've had very, very good luck with the bus service, I can only recall one time that I had to wait for an extrordinarily long time (over 30 minutes).