Who fulfilled the prophecy?
Who brought balance to the Force?
Anakin did and later Rey did with help from Anakin and the Jedi of the past
- As explained here, "balance in the Force" simply means no powerful Dark Side users perverting the natural flow of the Force.
- Balance in the Force can be achieved, lost, restored, then lost again etc.
- Anakin is the Chosen One spoken of in the prophecy, but balance isn't a permanent thing.
- One need not be the Chosen One of prophecy in order to defeat the darkness and bring balance.
So both Anakin and Rey brought balance to the Force.
Regarding the overall big picture, there does seem to be some room for interpretation. There's kind of two ways to look at it.
You could say
- Anakin brought balance in Return of the Jedi when he defeated Palpatine.
- But the balance was lost as the Dark Side grew strong in Palpatine and his followers.
- Balance was restored again when Rey defeated Palpatine.
You could also say
- It's all about Anakin being the Chosen One eventually restoring full balance.
- He ultimately brought balance in The Rise of Skywalker when he helped vanquish Palpatine once and for all.
This whole "chosen one prophesied to bring balance to the Force" idea is sort of a retcon. When George Lucas started making the prequels he wanted to justify their existence, so he started saying the prequels and the originals would all eventually form one big story about centering around Anakin Skywalker falling to and then being redeemed from the Dark Side. He wanted Anakin Skywalker to seem super important, going as far as calling him “the chosen one” prophesied to bring balance to the force.
The original trilogy however is not really Vader's story; it's the story of Luke Skywalker becoming the hero of the Rebellion and saving the Galaxy. Luke defeats the empire and the way he does so happens to involve redeeming Vader. Although he was an important part, it was never all about Darth Vader.
Even though each trilogy centers on a new protagonist, the overall Star Wars saga is still closely tied to the Skywalker family (which of course includes the Solo family as well). Whether on not "Skywalker" is literally their last name - Anakin, Padme, Leia, Han, Ben Solo, and Rey are all Skywalkers and the series is about their struggle to resist the Dark Side and bringing balance to the Force.
I elaborate on all this a bit more in this video: