Before you read any of this, be warned of major spoilers.
There are three endings to Bloodborne:
The I've turned into a Slug Ending:
Quoting this Reddit:
We become an infant great one, lifting humanity into it's next childhood, not much is said on 'lifting humanity into it's next childhood', it's a nebulous concept to say the least, but as a great one we ascend the limitations of humanity and can shape the dream world(s?) to our desire, good for your character, I guess, Gerhman is released from the dream, good, and Yharnam is finished which it would be either way, so no more hunters of the dream, good. This being the ultimately 'good' ending however is contingent on whether 'lifting humanity into it's next childhood' is a good thing or not, the only thing we have to go off of is that it sounds positive, to a degree.
To me, this sounds like you are beginning humanity's next step. This sounds like a good thing, unless the next step in humanity is bad, or the motives of the infant great one (now you) are negative.
The You died and you're released from the dream Ending:
Submitting your life wakes you up from your dream. But Yharnam doesn't seem like such a good place still. To quote again from that same Reddit:
It [Yharnam] hasn't a sustainable populace the chapel dweller pointed out, it's overrun by beasts and cosmic beings, it has an ancient city below it the inhabitants of which have a tendency to escape and capture people so they can fuse them together with beckoning rituals, there's a giant red moon hanging over it and there are gods hanging off of the spires.
The You've become Gerhman Ending:
Letting the Moon Presence "eat" you shows the player taking the place of Gerhman. You've now become the host for the hunters dream. What happens next is unknown. Perhaps another hunter will take your place in the future. Either way, the dream lives on.
Of the three ending, I would say the best/happiest is the I've turned into a Slug Ending because you are at least giving humanity a chance. This is also often called the "true ending" of the game by many.