I actually completed Unreal for the first time this year. That bit in the second(ish) level when the lights go off in that corridor and you hear the elite enemy before you see it (for the first time). Iirc the game generously gives you flares which provide a very small ungenerous amount of light making it even more tense.
That might have to be in my top 10 gaming moments which is quite amazing considering its age.