I'm partial to the conversation themes in DX1, especially the later ones. UNATCO, of course
its convo theme would be amazing to match probably the single best-known ambient track in the game or even the entire series, it's exactly what I would want to hear when conversing with my co-workers & superiors at a powerful, superficially good but shady futuristic agency. The
Paris convo theme sounds like the closest cyberpunk/techno-dystopian approximation to a track out of a medieval fantasy game where the world's been pushed to the brink & the peasants are crying out for a hero to save them (which is pretty much the absolute state of the DX world by the time you get to Paris). And
Area 51's theme does a great job at reinforcing the feeling that, as Paul/Dr. Savage pretty much spell out to you in their final conversation, the fate of the planet really does hang on your shoulders at that point.
The game itself aside, one of the things that impress me most about the DX1 modding scene (besides their insane passion & persistence in developing these mods, long after DX itself had been released and even Ion Storm had gone out of business) is their OSTs. These are way better than I would ever expect of community mods made by unpaid fans. For example,
the Moscow tracks from 2027 (a mod made by Russians, though they were kind enough to also translate it into English), very atmospheric & fitting considering those hubs are dark, rainy/snowy & crawling with hostile Russian military police:
Or Nameless Mod, its whole OST is full of bangers, but I particularly like the music in the last
couple levels, far better than could be reasonably expected of a mod with such a silly premise. The composers really brought their A-game at the end there.
Incredibly at least one of said composers, Steve Foxon,
was still actively making music as late as 2021. He had been such a prolific member of the DX community back in the day (IIRC he was also the composer for Zodiac, the older Paul Denton mod), and must've impressed even the staff working on the official prequels at Eidos with his various modding endeavors, that
he got referenced in Human Revolution.