The Way You Look Tonight by Frank Sinatra has been consistently cheering me up for… hm.. seven years now. It’s about the ideal woman, so it’s just appealing.
Anything by Bowling For Soup because of how quirky it is. On that note, any early rock and roll think American Graffiti soundtrack, too, except it mostly just sounds fun as opposed to quirky.
Here Comes the Sun by The Beatles because.. oh, if you know the song, you’ll understand.
Only the Good Die Young by Billy Joel because it emanates a youthful yearning for freedom. A feeling of breaking from the constraints of society and just having fun!
RAGTIME! Scott Joplin. Used in The Sting. It’s so light and fun and cheerful!
Also, when I’m exceptionally down, depressing songs cheer me up. Don’t have the slightest notion as to why, but I know that No Expectations by The Rolling Stones, On My Own from Les Miserables, and Wish You Were Here by Pink Floyd totally hit the spot. It’s actually very therapeutic, probably some sort of reverse logotherapy effect or something. I’m not a psychologist, so that’s an amateur’s guess, by the way.