Like everyone has said already, TW3's soundtrack is just fantastic and definitely one of my favorites. The track on my profile actually is from TW3 (Imlerith's theme).
Undertale's OST is amazing as well. I think that's the one I listen to the most honestly and is probably my favorite videogame soundtrack of all time.
Other OST that I really love and that have a special place in my heart are Persona 3's, Persona 4's, Kingdom Hearts' & Pokémon's, to name a few.