Sadly, I can only think of Stage 1 and 2 themes off the top of my head.
1: UFO's At the End of Spring. It just seems so light and happy, Zun really got that 'airy' feeling that he seemed to be going for. It's a feel-good song.
2: Imperishable Night's Song of the Night Sparrow ~ Night Bird. I don't know why, but I've always found that (at least on my Normal runs), the fairies have synced up to the first few seconds. Intentional or not, it makes it really memorable. And silly things that I've noticed aside, it's just a really pretty song.