SA-X Approaches… – I remember the first time I head this my heart stopped and I pretty much just stayed there until the SA-X left the room. That thing was terrifying on the first playthrough.
Escape from the SA-X – ohgoditfoundmeohgodohfuckohgodohfuckrunnningrunningrunningrunning
SA-X Boss theme – Finally getting to confront the SA-X at the end of the game once you were strong enough to defeat it was cathartic as hell.