All the music in Cars Land is definitely my favorite no contest, especially in the line for RSR!
Next would probably be Main Street/Buena Vista Street, and I also like Frontierland. Then there's Star Tours, which is also awesome, but that's just the Star Wars soundtrack, so I guess it doesn't really count...
I love all the music! I've actually become recently really impressed with the music and how it really makes the whole experience better and fits the mood of each place so perfectly. I recently rode California Screamin' when the soundtrack wasn't working and it made it way less fun. All the music makes such a huge difference!