First post was what immediately jumped to my head as well.
I want to nominate this one from the Neptune tank area. One of my favorite tracks of the first game / remake. Very haunting!
Edit: Ah shit, I overlooked Classicrock78's post. Sorry!
To nominate another one: the RE3 save room music. It's a bit calming, giving you a relief from all the horror lurking in the streets of Racoon City, yet it's also haunting, reminding you to not drop down your guard.