I'm assuming you are referring to ebooks only, so I'll have to limit myself to fiction (I buy nonfiction in hardcover).
It has been a real struggle lately to find 5-star fiction ebooks. But there are some. I would give all of
Vicki Tyley's mystery ebooks 5 stars. I would give all of
Shayne Parkinson's historical fiction ebooks 5+ stars.
In fantasy, I really like
Richard Tuttle. I would give most of his ebooks 5 stars, and none less than 4 stars.
Also worth 5 stars are Lee Goldberg's
The Man With the Iron On Badge and Jana DeLeon's
Mudbug trilogy.