My vote goes to A Christmas Carol. I know Dopers aren’t the biggest fans of Moffat, but I think the two best things he did for Who was actually include time travel in the stories and make the Christmas Specials about Christmas. A Christmas Carol is the best mash-up of the two. I think Last Christmas would have been the best if they followed through with the end and kept Clara an old lady. The rest of the story doesn’t really make sense with her coming back in the end.
Voyage of the Damned (10th) is the one I remember the most fondly.
I think the Snowmen (11th) is probably #2 for me. We had Clara as Mary Poppins. Gandalf as The Great Intelligence and Strax, Jenny & Madame Vastra. It was a lot of fun. I think that may have been the first time someone noted the TARDIS was smaller on the outside.
A Christmas Carol for me too. I didn’t like that season much overall, and Ten is by far my favourite Doctor, but that episode is perfection. The song is beautiful, Katherine Jenkins can actually act, the little boy was adorable, and Michael Gambon nearly made me cry.