So most RPGs that place even the tiny bit of emphasis on their music tend to have a "main theme". It doesn't need to be called that, nor does it even need to recurr a lot - but the game has a theme that is the very essence of what the game stands for and what feelings it evokes.
What RPG main themes do you think best illustrate what the game in question is about?
For me, there's a lot of those, but I'm gonna go with a relatively fresh example from my head. Final Fantasy usually handle main themes incredibly well, but this one standout piece may just be the single best illustration of the theme of the game in question.
It's also nice that elements of the theme appear in a number of other tracks in the game, really adding extra power to all those incredibly emotional moments. Definitely a modern classic.
Great theme, starts with a really good piano chord followed by creepy choral pieces and some percussion and guitar before the synthesizer kicks in to give it a real nice 80s sci-fi beat which sets well with a game that deals with some creepy trout set in a setting inspired by 80s cyber punk. Though if I am going to be fair, the series as a whole has always had great main themes.
The new entry has a pretty great central theme as well.