Even though I've watched it twice since TBOTFA came out, I don't know Desolation of Smaug all that well, because I had only watched it twice before that -- once in the theater, and once when I got the DVD. So this is a little bit tough for me. Also, those two times this year are the only times I've watched the extended edition, so bits of it are really new to me still.
One moment that has always filled me with breathless delight, though, is Smaug the gilded dragon. When he swirls up into the night, covered in liquid gold, I catch my breath in awe. The first time I watched it, when I got there, I said, "OH! Okay, all that stuff that felt too long before this was totally worth it."
Sadly, it's really hard to screencap his magnificence because he's moving really quickly the whole time. And still pictures don't do justice to how liquid-shiny he is.
Still photos also don't do justice to how cool he looks spiraling up into the sky and showering gold down on the camera and the world below.
Also, Bard's entrance is thrilling and perfect. I have a thing for silhouettes, and the first time I saw him standing above the dwarves there, I went, "Oooooh! Who is that?" One of the best entrances in both trilogies, I think.
And as for a favorite line, well... Cowboy and I quote Radagast a lot: "This is not a nice place to meet." Aside from that, I can't think of any I either love or quote a lot. Yet.