Number one moment for me has to be the
on the SNES, an absolute classic.
I'll never forget the feeling of isolation when you enter the space station and then discover the bodies in the lab, the atmosphere it generated was fantastic. Then having to escape when the place was going to self destruct, everything about it was awesome, the music, the way the environment moved, the sheer sense of panic. Completely breath-taking.
Second would probably be the battle against Meta Ridley in Prime 1, purely because it brought back memories from my first choice.