To judge simply by the artistic distance from the written source to the cinematic reworking, my top choice is The Black Stallion. The novel by Walter Farley was serviceable. Written when Farley was in high school and college, it’s a standard boy’s adventure with ordinary prose and better-than-average horsemanship.
In contrast, Carroll Ballard’s movie looks gorgeous, thanks to Ballard and cinematographer Caleb Deschanel. The plot is pretty much the same, and very basic it is. But there are real feelings of loss and risk, giving the triumphal story more layers. And the performances by Kelly Reno, Mickey Rooney, Teri Garr, and Hoyt Axton are wonderfully natural.