To whom does Victor Frankenstein tell his entire story?"Frankenstein" by Mary Shelley

There are many narrators (inner and outer) in the novel.  The first narrator is Walton.  He writes letters to Saville.  Walton, then, is the outside narrator.  You may say that he narrates the entire novel as all of narratives are filtered through him.  He is the "survivor" at the end who carries all the stories back to civilization.

Then, there's Victor, who tells his story to Walton.  He is the inner narrator, as most of the story is told by him, but again--it is relayed back to us through Walton.

Below is a table that shows the narrator / audience dynamics:

Narrators 1st Aud 2nd Aud 3rd Aud 4th Aud 5th Aud

Walton:  Saville Reader

Frankenstein: Walton Saville Reader

Monster: Frankenstein Walton Saville Reader

De Laceys: Monster  Frankenstein  Walton Saville Reader