For me it is always the Magic Kingdom Welcome Medley. If I hear "I'm walking right down the middle of Main Street USA" at any point during the day you can be sure that's what I will be singing all day long!
I love the Main Street Trolley Show. "Clang Clang went the trolley......ding ding went the bell......zing zing....when my heartstrings....fromt he moment I saw him I fell....."
I find "Small World" gets stuck in my head very easily so when I went to Disney a few weeks ago I decided I wouldn't go on the ride...I might as well have because I heard it playing everywhere I went!
The music from the Tapestry Of Nations Parade that they had in Epcot, has been getting stuck in my head since 2000. That parade was great, I wish it would return. They still play the music in Epcot in the World Showcase after Illuminations.