Well, that's a tough one. 'Gamera' has a pretty decent anime adaptation. It manages to capture the monster's power and the story's excitement. But opinions vary as different people look for different things in an adaptation.
The manga and anime are closely related. The anime usually tries to stick to the main plot and characters from the manga.
For me, Naruto's anime adaptation is top-notch. It manages to capture the intensity and emotion of the manga perfectly. There are a few fillers, but they don't ruin the overall experience.
For many, 'Death Note' has a top-notch anime adaptation. It stays true to the core concepts of the manga while adding its own visual flair and narrative enhancements.
I'd say 'Fullmetal Alchemist: Brotherhood' has the best anime adaptation. It sticks closely to the manga, has amazing visuals, and the voice acting is superb. It's a near-perfect translation from page to screen.
For me, 'Naruto' has one of the best anime adaptations. It keeps the key plot points and character developments from the manga intact while adding some nice visual enhancements and filler arcs that enhance the overall story.
One could argue that 'Fullmetal Alchemist' has an excellent manga adaptation. The anime closely follows the manga's plot and character development.
I think 'One Piece' has a great anime adaptation. The characters and storylines are brought to life really well.
In my opinion, 'Attack on Titan' has a great adaptation. The anime captures the intense atmosphere and complex storylines of the manga quite well. There are some minor differences, but they don't take away from the overall quality.
One could argue that 'Fullmetal Alchemist' has an excellent manga adaptation. The anime closely follows the plot and character development of the manga.