while i thought the build up to TBC was pretty good, and first stepping thru the portal seeing all the demons fighting with both alliance and horde npcs was nothing short of epic, i really have to say that my favorite was probably the flight into mt. hyjal.
in reality, given all its shortcomings, Cata for me was actually pretty decent from a story perspective and i actually felt like the world i was traveling around in was in peril. the flight into mt. hyjal gave me a pretty big sense of "damn shits fucked up here and looks like its getting worse"
to each their own though.