What is an example of an adaptation of a book where the TV series was better?

Neverending story.

I enjoyed the first few seasons of Dexter more than the books.

I'm going to go out on a limb here and say Foundation.

I love the books, they're legendary, but the show on its own, completely separate from the books, is a great watch. The books are almost like reading a very long thought experiment, it's more philosophy than story. There's also a lot of "it was a different time" cringe in the books that does not age well.

The Magicians but only for the first three seasons

Killing Eve. I loved the TV series in its first season, and excitedly grabbed the original novel because I'm usually one of those "the book was better" people. I was so disappointed.

The book is just a standard spy thriller, a bit on the dull side, with one-dimensional characters and a meandering plot. In the book's version of the world Eve is after Villanelle not because of the appealing self-questioning or confused obsessiveness Sandra Oh's Eve displays, but simply because she's the good-spy character who is angry that her colleague was killed by the baddie. The book's Villanelle is not the badass mischievous psychopath infatuated with her pursuer as Jodie Comer plays the role, but just another unexcitingly-written assassin who has some dull sex scenes with strangers in between killing people and musing over her tragic past of killing people. The barely-written-at-all supporting cast are also nowhere near as interesting as the TV versions; prepare for the thrilling adventures of stock spy colleague, stock hacker kid, stock assassin guy, stock Chinese person, and so on. The book is Killing Eve minus anything that made the TV series interesting.

It's obvious that while the novel was used as a starting point, the real charm of Killing Eve came from Phoebe Waller-Bridge, Jodie Comer, Sandra Oh, and the rest of the cast and crew adding real depth, spark, charm, and appeal to the characters and settings. This novel was basically the rough-draft pencil sketch of a masterpiece painting, better completed by others.

"Legend of the Seeker".

The show is bland fantasy tropes stitched across the best ideas from the book.

From my own understanding of the novels reputation: Babylon Berlin.