I thought this was an interesting idea to pursue separately, what movies are better than the source material? We can include comics as well.
While a fan of it (and I read it before the film was even announced) I think Trainspotting is probably better than the book which is a bit uneven because some short stories are cut and pasted into it. The way they added more humour while still confronting the horrors of the subject matter was a fantastic balancing act.
I think Harry Potter and the Deathly Hallows is better than the book, it’s much better paced as the book frustrated with spending too long camping in the woods and not getting to the big climax of the series.