Star Destroyers in X-Wing/Tie Fighter. It was fun to scramble around them, dodging turret-fire while dogfighting swarms of TIEs, just to deliver a couple of heavy rockets on their shield generators. Ok, it was fun while piloting a TIE Defender.
Ah, and that painful disappointment as a Star Destroyer jumped out of hyperspace right through your fighter... talk about being in the wrong place at the wrong time.

Bugbeasts and Haunts from Thief 1 were my worst gaming nightmare of all times, anyway.

Adolph Hitler in Wolfestein 3d Episode 3, loved the way he would disgregate in a crimson pulp of blood and bowels.