This was hard to pick out just one. But the one move that stood out was Haku's Crystal Ice Mirror attack.
This move has been my favorite for a long time even though I don't care for ice base moves. Plus I always used this move in most of the naruto games ( when I play as Haku).
That's day 23 one of the hardest one. next is day 24 moment that shock me the most in any anime.