I've had this idea for awhile and was surprised it had never been discussed here before!
One of my favorite details in any theme park is the music they play in the background as you're experiencing each area and attraction! There are so many great area loops to pick from, both iconic and obscure. I'm excited to see which ones you all choose to bring up!
I think one that gets slept on in fan communities is the DHS Toy Story Land loop. For a franchise that only dips it's toe into being a musical, there is an incredible amount of iconic and instantly recognizable musical pieces and motifs that have spawned from it. This loop utilizes that wide breadth of nostalgic tracks in really interesting and energizing ways, and I love the ways that it composes even the slower and more somber songs like "I Will Go Sailing No More" or "When Somebody Loved Me" into a peppy, lighthearted soundtrack. One of the best things to come out of this land imo.
One of the most iconic loops out there is this one from MK's Tomorrowland. I believe for good reason too, the synth soundtrack sounds incredibly futuristic and fantasical, while never dipping too far into synthwave or overly nostalgic, 80's-y tones. Utilizing both original compositions and arrangements of songs from movies and past Tomorrowland attractions, it's one of those loops that always brings me back to the feeling of walking around the land itself.
The loop from IoA's Marvel Superhero Island has always been one of my favorite parts of that land. Very 90s-y, action packed, it fits perfectly as the backing track for all of these animated heroes and villains duking it out in the streets. I think a number of the tracks in this loop give such a fun summer-y vibe that just makes any especially hot Universal park day a little more bearable.