Born This Way - Lady Gaga
I've heard this song once in my life, and I already feel like I've known it for years, and it's been going through my head for days.
This sucks. Yet it's a testament to the inherent catchiness of her music compared to a lot of other pop music which only gets stuck in your head because pop radio stations play the same 4 songs on repeat until you're ready to take a power-drill to your temple to get the damn songs out of your head.