I have to say, any fade/Usurp of the fire ending.
The first game was amazing with it's Bowing serpents as you walked out of the Kiln.
The Second returned to greatness with (SotFS version) that scratchy narration and the flames lighting and fading.
Then the BEST (In my opinion) is the betrayal ending in 3 after giving the firekeeper eyes.
She agrees to forsake her duty and stay loyal to you. Even to help you end the age of fire and let it fade.
Even as light ends and the fire burns away to nothing... She stills calls out to you. Asking if you see the tiny flames flickering.
It always sends shivers up my spine. And I have to agree with BabyYoda that the audio is what makes it so amazing.
Even the shrine gets a different time and music when on way to this ending. And as a whole. Being able to put down the flame that's burned away so many. It makes me smile.