Okay, i'm going to be the black sheep and say "All Star Music"!
Why? Well personal taste i guess...i just love it. It has everything i need ( a bed and a shower!). No..seriously, besides that i like the themeing better then the 'louder' thememing the other 4 have ( not a huge fan of giant statues...lol...).
Most here have mentioned dedicated bus service. See, for me...that is one of the down sides to staying at Pop. With the All Stars, no matter what resort you are at, ultimately you have THREE bus options. Pop you don't...you have to wait in line until the dedicated 'Pop Only' bus arrives.
Far too many times i have had my butt saved from being left standing in gigantic lines for Music by simply hopping on the Sports of Movies bus instead. Sure it means i will have to walk 10 mins or so across the other resorts' parking lot to get to Music...but that sure beats still being stuck in line after Park Dump! Of course, this is me alone...i am not with a family or kids in tow, so again, this would probably not work for some visitors. For me though, it's a lifesaving technique when things get backed up at the bus depot after the Park closes and the lines are completely insane.
So part of Musics' appeal is that: More bus options if need be.
Also, Pop is out in the middle of nowhere. There is nothing around it, and it seemed to take longer to get TO and FROM the Parks. I will always remember my first bus ride back to Pop from a Park...it was like 'Dang, are we still even On Property? Again, this was just my impression. At the All Stars, you feel like you are in the thick of WDW Property as you can readily see things close by. With Pop it seemed too far off the trail...but again, that probably appeals to some folks which is cool.
For my needs, Music is awesome. It also tends to have the lowest amount of children when compared to the other three Value Resorts ( they must all like the big statues...lol!).
Again, just observations. I can't get on the Pop bandwagon.....