It all really depends on my mood.
If I'm happy I usually have a Beatles Song stuck in my head (usually Hey Jude, or All You Need is Love)
If I'm feeling romantic (don't ask) then it's always Tom Petty. American Girl, Running down a Dream, and Free Falling are some of my favorites to get stuck. Her Comes My Girl a more recent one.
If I'm feeling sad It can vary, mostly it's The Who Behind Blue Eyes or Girl (the Across the Universe version, only because it doesn't have an up beat drum beat, so it's more depressing) another Beatles tune.
And I have been known to get these tunes stuck as well: Won't Get Fooled Again, An American Band, I'm Sailing Away, Bohemian Rhapsody, the Animal Crossing 4:00 theme, Happiness is a Warm Gun, Stairway to Heaven, the American Beauty theme song, Sweet Emotion (thank you Warren and GH: Aerosmith D:< :D... ?),Time and Brain Damage/Eclipse, Babba O'Riley, Piano Man, Rocket Man, Imagine, and MANY MANY others.
Marching band music. 
Right after practice amirite? Though you were blaring Jackson Five on my youtube today...
So obviously, if you couldn't tell I'm more of a classic rocker myself. All you're new age crap can suck my balls.
