Not a fan of their OSTs (especially when they were often compared to the Silent Hill series back in the day; inferior in every way IMO) but I really have a nostalgic feeling everytime I listen to RE2 and Outbreak. RE3 not so much but 2 was definitely one of my favorites in terms of the more formal arranged pieces (like the one at the hall in the police department).
Close tie between CV and 2, but I'd have to say CV. Has a lot more songs that are actually like songs and much more orchestral and grand. 2's main driving points are things like the Third Malformation of G (which I've NEVER understood why they left out of Darkside since it's practically a main theme of the game) and the Self Destruct Sequence and the final fight with the T-103 and stuff.
I would've said Remake, but that more or less has ambient noises as melodies (which is good, just not as much of "songs" as the others).
Oh, and no one ever forgets the Outbreak Theme and Third Time's the Charm.
Another honorable mention has to be Nemesis' final fight theme.
Before you die you see the Tails Doll---Backdrop Observer of the Metroid: Other M board Apparently Edward: Wise Old Sage of Korodai