okay that's a hard one. i keep discovering new songs and bands that i absolutely love, there's many songs i come back to enjoy years after as well. i can't really pick my #1 so i'll give you 3 that i actively listened to for years (which aren't my favorite songs at the time though, check my pedi to see what i listen to atm!):
Jester's Game (The Garden)
Black Glass (Machine Girl)
Brand X (Nero's Day at Disneyland)