The Vault. Beautifully designed (especially the Inner Sanctum), fun boss fights, good music, great atmosphere - I love the hushed, serious tone the place has that really sets it apart from other typical dungeon runs, and adds to the feeling that something big/ game-changing is going to happen.

Shiva is my favourite trial for the same reasons, as well as the cutscene before the fight (one of my favourites in the game) and of course, Diamond Dust.