One of my favorite expansions of all time is Wrath of the Lich King. I was at the midnight release of the expansion at a local gamestop and I remember them playing the Cinematic trailer before opening to make the moment all the more epic.
This trailer is my all time favorite trailer for any game ever. Why? Well, the visual for one is incredibly stunning. When I watch the trailer, my room feels just a little bit colder. I FEEL as though I'm witnessing the rise of the Lich King in person.
But the most unique thing about this trailer has to do with the story that is being told from the Narrator (Arthas' father). Arthas' actions throughout the trailer end up mirroring what his father is narrating. It gives a nice effect of story, by showing Arthas' downfall by comparing him to what he should have been.
All in all, it's just a really well designed cinematic and I've watched it fifty times by now.