The Death Star run from the original X-Wing was all sorts of epic, as your ship goes in the trench and then triples its velocity! Epic!
Yeah, loved the Death Star levels in X-Wing, including the laser tower clearance level and the preceding massive space battle.
I also have a soft spot for some of the other X-Wing levels, including the training mission in an A-Wing where you have to fight other Rebel fighters in an arena-like mission.
And of course, for me the single best moment from TIE Fighter:
"Enough of this charade, Gamma 1 is the Emperor's stool pigeon! Gamma 2 and 3, destroy Gamma 1 now!"