Edit: 20 upvotes and 89 replies in one day! I will give my opinion on the best soundtrack.
[spoiler]all of them[/spoiler]
Edit 2: [spoiler]All of them except Nessus[/spoiler]
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Either Deep Stone Lullaby (I know, predictable AF - idc) or the Riven encounter theme. Favorite raid encounter in the game, riven legit, also has my favorite OST. It slaps.