It's a toss-up between Code Veronica and RE4 for me. I'd probably lean more towards RE4 because it has some great, creepy atmospheric tracks. I really like the save room theme and the Mercenaries themes as well.
My single favorite track from the series though is the original save room theme. Nostalgia really hits me on that one. It was a nice touch when they used it for Lost in Nightmare's results screen.