The Witcher 3 OST is awesome in general. I've seen a translated version of this one here: https://www.youtube.com/watch?v=gMB92nmbGxk
Ladies of the Woods is another favorite, too: https://www.youtube.com/watch?v=SiyOajnJnTg
My two personal favorites are two tracks which are not part of the "official OST" because they're dynamic:
Forefather's Eve: https://www.youtube.com/watch?v=6AtXx_8c4Wg
And (my overall favorite) the combat theme for White Orchad and Kaer Morhen: https://www.youtube.com/watch?v=w1x-wJlbf3g
Most (I'd like to say all but I think one or two aren't, not sure) of the songs from OST were made by Percival (the "traditional" branch of Percival Schuttenbach... they named their band after an character from Witcher saga, it's all connected
) years ago and only got a bit edited, the dynamic ones are usually made specifically for the game.
They're either actual ethnic Slavic music (I'd say 50-50 when it comes whether it's original Pagan or later Christianized versions) or based (you can see it straying from "originals" especially in the ones made for game) on it. Funilly enough the official classification is folk metal despite that sometimes their songs are literally just recorded versions of what existed for hundreds of years. They even perform those on period-accurate instruments on various Slavic Festivals and stuff...
Yeah, say what you want but back then we had the best music.
I am still waiting for the newest album of Percival. They already made East Slavs and Southern Slavs, now they're doing Western Slavs which means Poland.