It's a hard decision for me, I feel like RE remake's save room music is the best when it comes to presenting the perfect atmosphere of dread lurking around the corner while providing a sense of brief safety. It's very dreamy and mysterious, which fits the game as well.
But if I have to go by pure emotions, I think Code Veronica X's save room music is by far the most beautiful. Its the only save room song in the series that sound distinctly Japanese, and it reminds me a lot of Final Fantasy and tragic anime music. In a way, it sticks out since it's not very traditional RE style. Granted, RE0 echoes Japanese style as well, but not to the same extent.
RE2 and RE3 have very similar save room themes, mainly because of their inclusion of piano strings and similar synthesizers. They're both equally good IMO.
I also want to give a special shoutout to the result theme from Revelations 2, its simply a very good and creepy theme.
The original RE2, closely followed by 3. I love how both tracks sound so sad and desperate, perfectly capturing the state the city is in at the time, while still making you happy to hear them because it means you're save... for now.
REmake 2 has a second version of the save room theme that plays when you're looking at the unlocked figurines in the main menu. I like that one too. The one in the game itself, not so much.