If your soundtrack is in the Age of Empires series, please also try to list a soundtrack in another series as I feel we’re probably going to get a lot of AOE games as answers.
In the age series I feel its close between the base game of Age 1 and 2. I love them both so much and they’re so steeped in nostalgia for me its hard to pick one. I feel like 2 edges it out slightly but only ever so slightly.
Out of the Age series it might be Halo Wars or Command and Conquer: Tiberian sun. Halo Wars is a mix of both Stephen Rippy and the beautiful Halo soundtracks which is great on all levels. I only finished Halo Wars last year but the soundtrack has to go down as one of my favorites.
Tiberian Sun is a great mix of the intense upbeat Techno/Rock work that Frank Klapleki is so well known for as well as hopeless dead ambience and a strong grim atmosphere which I can’t really disagree with. The perfect mix of a thick mood to pull you in and high energy to keep you going.