I didn't realize that the poll was for 3, so along with A Christmas Carol I added The Time of The Doctor and The Husbands of River Sing. Both are good episodes (THoRS a bit better then TToTD) that really stand out by having a few spectacular moments (I doubt any Doctor will have such an emotionally effecting, for me at least, regeneration scene as 11 at the end of TTotD and River realizing 12 was The Doctor was just great).