The gentle thrum of a Federation starship's warp core.

My cat purring.

thats hard but my top three is a light rain storm so just a bit of far away thunder, waves, and campfire. I have found some recordings of a compfire on a beach in a rainstorm and its just the best.

The save room music in resident evil 4.

Jokes aside my most relaxing sound is heavy rain on a tin roof. Followed by cats purring.

It's a tie between the sound of heavy rain at night and that strange silentness you hear when it's snowing heavily, the kind that you "hear" even inside the house which makes you want to go and check if it's snowing!

The sound of rain on a tent mixed with a crackling fire, very relaxing.

The sound of snow.

A fire crackling in a fireplace. I'm also quite relaxed by heavy rain.

creaky wooden ship on stormy seas or rainy day steam train noises are so nice for me

The soothing tones of the dark souls 1 character creation menus

Partner's giggle

Moderate rain and distant thunder.

The sonic charges in Star Wars Episode II

Rain outside my window in the garden. Maybe not the MOST, but it's up there.

Cat's purr

Light rain on leaves

Locally, indoors? Silence.

I like to be able to hear whatever is going on outside. Ideally, that would also be silence, but if something is going on outside, I like to be alert and aware of it.

I'll also take a little rain and occasional thunder outside, as usually people aren't acting a fool outside during rainy weather.

Crashing waves at the beach. Seagulls are little assholes but they add to the effect for me

Probably a soft Rhodes keyboard when it comes to something more dynamic, but other than that I'd be tied for the sound of rain and the sound of waves.

A babbling brook.

White noise of a fan. I always have one on when sleeping so hearing that sound reminds me of a cozy nap.

Train whistle and the sound of the train rumbling by in the distance is deeply relaxing, doubly so if after nightfall or if I'm drifting off to sleep.

Currently:

https://www.youtube.com/watch?v=APpkBhWeoFk

Train whistle in the distance. Light rain and thunder.

I can't decide, there are too many:

Mongolian throat singing with little to no instruments.

The sound of a plane cabin (I think its brown or pink noise)

The fire crackling of a fire place or furnace.

Ocean waves or waterfall or river noise.

Slowly and softly played piano.

A stew slowly cooking.

Sound of gentle brushing your head hair.

my favorite asmr triggers are tapping, scratching, wood, and slime

Rain and thunder outside while inside in the evening.

Also, due to occupational damage, the sound of an offshore seismic source going off in the distance ever 10-12 seconds. The sound and the low frequency shake of the ship means everything is working as it should.

I don't know about most, but generally those "background sound" sort of ambient things are pretty relaxing.

goes looking on YouTube for some examples

Rain: https://www.youtube.com/watch?v=8plwv25NYRo

Lake waves: https://www.youtube.com/watch?v=Se5NjX-cM5I

Ocean waves: https://www.youtube.com/watch?v=WHPEKLQID4U

I think that the shakuhachi, a Japanese bamboo flute, is relaxing to listen to.

finds an example on YouTube: https://www.youtube.com/watch?v=miUKO5g0ONk

the sound of capitalism within the apple app store