I'm a big fan of when the in game music fits well with the game situation, and changes accordingly.
Doesn't really matter to me what genre of music tho, as each seems to fit quite nicely in different situations. Heres a few that stuck out to me:
Cypress Hill in kingpin, needs no description, fitted with the game really well.
The Victorian-esque string quartet from Arcanum. again, seemed to fit the world well and I liked how it changed when you were in different places or when someone attacked.
The 90's metal in final liberation, sounded great to move in an army of space marine tanks with that behind it!
and by far one of my favourites. the grand orchestra of Jeremy Soule in total annihilation. Genuinely perfect for the atmosphere when it switched between the slow, calm music of building and exploring to the huge, cataclysmic war music when you get attacked by the enemy from all sides.
I’m gonna go off on one for a sec now :D
For any of the music buffs out there, check this game out. To me, when the TA war music kicks off I get a great image in my head, the higher pitched contrapuntal melodies interweaving with each other are like the small planes and scout kbots trading fire, then the booming tubas and bassoon start like the huge powerful behemoths and artillery of the army slowly pounding through the battlefield while the smaller units fight around their feet.
The mans a genius, I think it really captures the grand scale of what the game was going for.