As the title says, I want to know if there are any anime adaptations (of manga, light novels, visual novels, ...) that significantly improved upon the source material. I'm sure they're out there, but as someone who doesn't really read like that, I'll never consume enough to know for myself.
Chuunibyou demo Koi ga Shitai! (Love, Chunibyo and Other Delusions). People say that Kyoto Animation pulled off a miracle with K-ON, but if you actually look at the K-ON manga, it wasn't bad. It just needed fleshing out, and it also helps if you can actually hear the band and not just read about them.
Chuunibyou was KyoAni's real miracle, though. If you try reading the original light novels after watching the anime, you will be shocked. Pretty much all the elements that made the Chuunibyou anime great were KyoAni original. That includes the characters Dekomori and Kumin, as well as Rikka's backstory. In the light novels, Rikka is just a girl who's quirky for no real reason, but KyoAni made up a really good backstory about her origins as a chuuni.
And besides that, Nibutani has an even more abrasive personality in the LNs (pretty much all tsun and little, if any, dere), so she's not likable as she is in the anime.
So, the Chuunibyou LNs gave KyoAni straw to work with, but they managed to spin it into gold.
The first season of AoT 100% improved the manga for the better. Isayama has openly stated he took direct inspirations from the anime adaptation to improve upon the characters and plot of the manga. The anime also made small changes to the story while also introducing a new brilliant art style and beautiful sakuga that complimented the series. Attack on Titan's manga wouldn't be where it is now without the anime adaptation.
86 Eighty Six
this is the only one I've seen that I'd say was significantly better. In general every LN I've read has really bad prose. Maybe they were fan translations and a lot of the nuance gets cut out. Or more likely, they're cheap disposable stories sold at train stations, the equivalent of Tom Clancy or Dean Koontz. Fun and no substance. whereas the 86 anime (to me) took significant advantage of unique aspects of the genre (& I am a Sawano fangirl).
i need to be reading more good novels in general