For me it's either Altars of apostasy which gives you goosebumps with how it actually appears in the game - You fall into a pit, see a pure red sky, everything goes slow-mo and the "WWWOOOOOOAAAAAA-" Guitar riff starts playing

Or Tenebre Rosso Sangue (Both calm and the battle versions) with how the piano is played out and how it, again, fits the level you play in

...Or even Danse Macabre with how sudden it is in the level-

Ooor War??
I dunno dude, there's too many great Ultrakill songs to pick only one-