Easy -- Sakuragaoka Joshi Koutougakkou Kouka (their school alma mater song). Because despite all the flash of pop and rock & roll and fun... K-ON is a story about how girls became friends and went through high school (and beyond) together. High school is a door into another you you're going to be, so for me this gorgeous, simple melody is the foundation upon which our HTT girls are standing. (^- ^ )