The IVS-Kran was a good one, as well as Bluff Eversmoking. The level that really terrified me was the first one, when you first get to see the Brute, he's charging out at you, dual automags blazing. Or your first encounter with a skaarj, when all the lights go out.
Goddam, that game was so cinematic. THATS WHY I LOVED IT SO GODDAM MUCH. Ha, I almost forgot why it was so dear to my heart. It wasn't just a run and gun action music, open the door fire your guns cause a monster's waiting for you. The pauses were dramatic, enough to build up fear and tension. I played through DooM, Heretic, Hexen, Quake, Quake 2, and they were all ho hum. I always knew I'd go around a corner and something would be there... but with Unreal, it wasn't predictable. The lights were so ambient and dramatic, the moods and tensions so perfect, the theme so ironic (the damned becoming the savior). It was just a beautiful, innovative play experience. Single player unreal is definitely something I'm going to play through again. That whole game scared the mortal fuck out of me, and no other game can stake claim to that. I remember creeping in terror through the Tyridium mines with the stinger in hand, not knowing when the next mercenary would spring up on me.... *drool*.
On a side note: The icon for the red team in UT (the red lightning bolt-ish thingy) is the symbol for tyridium from the original game.
Its pretty funny how the whole first game virtually revolved around tyridium, and the second game's only (fleeting) reference to it is the new GESBioRifle, that has tyridium after-product.
I also thought it was beautiful how Epic and de made their online play game coincide with id's, and delayed the actual sequel, knowing full well it would kick its ass just like Unreal did to Quake and Quake II.