For me, it's Bonetrousle.
The night I heard it, I couldn't stop listening to it. I practically danced to this song, and by the time I noticed, it was like 4 AM.
Since then, I've listened to it almost daily. It brightens up my mood, and I don't know why. Heck, I've even made it my RINGTONE and ALARM, and I haven't gotten bored of it.
God bless Toby Fox for this masterpiece.