Animation, as is the actual animation bit, or do you mean our best brickfilm?
And best as in most well received, or best as in the one we are most proud of?
I know this could easily turn into a shameless bragging thread, but, for now, I like the idea. Seeing what people chose as their favorite personal film may be pretty cool, especially if it clashes with what we may normally think. The idea sounds familiar, but I couldn't find if we have ever done this before.
And assuming you mean the brickfilm that we are most proud of, I'd have to say: And Men Loved Darkness.
It isn't quite as polished and compact as some of my others, but it is by far my most ambitious, and I'd like to think it turned out pretty well. It has a strong moral message that is complemented well by the great visuals and most complex and layered sound mix that I'll probably ever attempt. The script could have gone through another draft, but otherwise, I'm really proud of the result.
Edit: The more I look at it, the more it seems like you may mean the actual animation bit.
If so, for whatever reason, I'm really, really happy with the way this turned out. It's fairly simple and short, but the physics turned out fantastic.