I'd say It's A Small World as well; except I'm NOT kidding!
Music from the Disney parks and animated classic films is literally about 50 percent of what I listen to, so this is a subject VERY near and dear to my heart! Music plays a HUGE role in how I experience the parks, and what I remember most about them!
I can't possibly pick just one thing, but if I HAD to narrow it down, my "short list" of favorites would include every Fantasyland attraction soundtrack and music loop, the Toontown music loop, the Splash Mountain, Country Bear Jamboree, and original Tiki Room soundtracks, the original and current Imagination soundtrack, the Nemo and Gran Fiesta Tour soundtracks, the It's Tough To Be A Bug preshow loop and theme song, and last, but certainly not least, the Main Street Electrical Parade and Wishes soundtracks!
A more comprehensive list of WDW attraction soundtracks and music loops I love include, but are not limited to:
Magic Kingdom
It's A Small World
Winnie the Pooh ride-through and area loop
Snow White's Adventures
Peter Pan's Flight
Mickey's Philharmagic soundtrack and queue loop
Mickey Mouse Revue soundtrack (defunct)
Cinderella's Golden Carousel loop
Dumbo the Flying Elephant loop
Mad Tea Party loop
Pinocchio Village Haus loop
"Merilly Song" from Mr. Toad's Wild Ride load area (defunct)
Mickey's Toontown Fair loop
WACKY Radio loop (from Minnie's House)
Splash Mountain ride-through and queue loop
Country Bear Jamboree soundtrack (for all three shows)
Big Thunder Mountain Railroad area music
Enchanted Tiki Room (original show and parts of the current)
Tiki preshow area "Tiki Lounge music"
Swisskapolka (from Swiss Family Treehouse)
Magic Carpets music loop
Yo-Ho (A Pirates Life For Me) from PotC
Adventureland main music loop
Carribean Plaza steel drum loop (now heard only from a few speakers near the usually closed El Pirata counter-service area near Pirates)
Carousel Of Progress Soundtrack
Tomorrowland music loop
Star Tunnel entrance music from Space Mountain (heard here!
http://www.youtube.com/watch?v=F975mpnbg-I)
"Galactic Anthem" load music from Stitch's Great Escape
Sonny Eclipse songs from Cosmic Rays
If You Had Wings theme song (defunct)
Main Street Music loop
Welcome Show and Trolley Show
Main Street Electrical Parade
Wishes
Spectromagic
Main Street Christmas Loop
Holiday Wishes
Once Upon A Christmastime parade soundtrack
Fantasy In the Sky (NYE fireworks show)
Celebrate the Season Castle stage show
Here are a few musical favorites from the other three parks:
Epcot
Journey Into Imagination "One Little Spark" theme song, ride score, and area loop (current and original versions)
HISTA load music (defunct?)
"We Are Here To Change the World" and "Another Part Of Me" from Captain Eo
"Big Blue World" and score from The Seas with Nemo and Friends
Soarin' score
Kitchen Kabaret soundtrack (defunct)
World Of Motion soundtrack (defunct)
Universe Of Energy score (current as well as original defunct songs)
Innoventions area music
Gran Fiesta Tour soundtrack
El Rio del Tiempo songs/score (defunct)
Mariachi Cobre
Spirit Of Norway load music and score
Spellman's Gledje (defunct)
Reflections Of China score/load music
Si Xian
Octoberfest Musicanten
Impressions de France score
British Invasion (especially when they play selections from late-period Beatles, such as the Sgt. Pepper, Magical Mystery Tour, and Abbey Road albums)
Music loops from all 11 countries (favorites are Mexico, Norway, China, Germany, and Japan)
Illuminations preshow loop and main show soundtrack (especially the extended Christmas version)
Tapestry Of Nations (played in it's entirety around the lagoon after Illuminations)
Animal Kingdom
Oasis Music Loop
Tree Of Life Music Loop
Discovery Island Music Loop
It's Tough To Be A Bug preshow loop (can be heard in it's entirety here, with no crowd noise! -
http://www.youtube.com/watch?v=of16p1xPiKc)
It's Tough To Be A Bug theme song
Festival Of the Lion King soundtrack
"Colors of the Wind" from Pocahontas and her Forest Friends show (defunct)
Africa Music loop
Habitat Habit music loop
Flights Of Wonder score and preshow music
Asia music loop
Everest music loop
Finding Nemo: The Musical soundtrack
Mickey's Jammin' Jungle Parade
Mickey's Jingle Jungle Parade
The Tropicals steel drum band
Disney's Hollywood Studios
Fantasmic! Soundtrack
Magic Of Disney Animation area music
Walt Disney: One Man's Dream area music and film score
Beauty and the Beast musical soundtrack
Voyage Of the Little Mermaid soundtrack
Playhouse Disney: Live on Stage soundtrack
Star Tours area music, "droid rooms" music, and ride soundtrack
Great Movie Ride lobby music and ride through music (fave parts are the Mary Poppins and Wizzard Of Oz parts)
Honey I Shrunk the Kids Movie Set Adventure music loop
Muppet Vision area music (especially "Rainbow Connection")
Stars and Motor Cars parade soundtrack (defunct)
Osbourne Festival Of Dancing Lights soundtrack
Here are a few of my favorite pre-existing tracks not written for Disney or recorded for the parks, but used in WDW music loops (aka "needledrop") that I wanted to know the name of for years, and finally do now:
"Der Kinderlein Kommet" ("O Come, Little Children"), from Music Box Christmas, from the Main Street Christmas loop:
http://www.youtube.com/watch?v=eEXlc3IeSCE
(from 2:15 to 5:15)
"Christmas Canon", by Trans Siberian Orchestra, from Osbourne Lights:
http://www.youtube.com/watch?v=nzqQdc8wljY
"Behind the Waterfall", by David Lanz and Paul Speer, from Honey I Shrunk the Kids Movie Set Adventure:
http://www.youtube.com/watch?v=avAa5TA5joc
The stunningly sublime "Out Of Africa" main theme, by John Barry, used in both the Great Movie Ride lobby and The Land area music loop:
http://www.youtube.com/watch?v=FSO3DEcyxPI
"Pahari" and "Raghupati II", by Jai Uttal, from the Discovery Island Music loop (can be heard in their entirety here):
http://www.ufomusic.com/product.php?id=140
