What games have you played that have a "perfect fit" soundtrack?
Many games feature amazing music, but certain games take it beyond even that.
Games like DOOM are known for the "procedural" composition they use to marry gameplay and sound, and not only that, the way the music is a perfect tonal match to what is happening.
What games have you played that feature music that doesn't just make you notice it, but also pulls you further in?
Most modern games do this. It's a little different depending on implementation, but usually there are short loops of music which can transition into one another, and the game attempts to detect what's going on and make smooth transition at the end of each loop into an appropriate new loop based on the situation, so that the music is seamless but still reactive to what's going on. When it's done well, it's basically invisible that anything special is even happening, which I'm sure is irritating if you're the one who had to do the ball-busting labor of getting it all to work.
Fun fact, LucasArts was already doing this all the way back in 1991, back when video game sound beyond the bleeps and boops stage was still bleeding edge technology. One of a few different ways in which they were ahead of the curve by about 20-30 years if not more.
The wiki article goes into it; the canonical example (if you know to pay attention to it) is walking around in Woodtick in Monkey Island 2, and you can hear how the melody that’s playing is continuous, but some of the backing instruments will insert or change depending on which buildings you go into. You can probably find Let’s Play of the game on YouTube or etc to hear it in action.