Itokubo's Onegai Kamisama
It's stuck in my head.
Though it also reminds me that if Keiko got from there to where she is now I can too (I'm resuming singing lessons next term with a new teacher, yay.) It's really cool that each Kalafina performance has a chance to be lesser than it's predecessor in a certain way. It reminds you Kalafina are human, not robotic and just like us they have their off days.
Also the editing in that PV is hilarious (the movements too.) I've spent so many hours editing 30 second clips of what can only be described as absurdity, not to mention my teacher's holiday footage for an assignment. So, to see something professionally done which leaves a slight white border around a png image humors me (though honestly Kalafina promotional art is just getting worse, I could do a better job.)