I much preferred Eragon over HP films (as movies). I actually don't understand the negative reception of the Eragon film overall. It had a lot in it that I had been wishing to see in Fantasy movies and is almost the only one to deliver (well)--excepting LOTR and Hobbit. I agree that they messed up the adaptation in important ways including characters, plot, and some details (like the races), but those things could have been added/changed without fundamentally changing the film as it stands. If you look at the movie separate from the book series, it's actually not so bad. It had its share of lame moments--almost every movie does, but I feel this was within industry average tolerances. And I think the HP movies wouldn't be all that great as standalone movies without the books to back them up. They're not awful, and the adaptation was great, but I just didn't love watching them and probably won't see them again, whereas I've rewatched Eragon several times despite its flaws. So if you're talking about how well they adapted the book to film, I totally agree with the concern, but for the most part, Eragon was not a bad movie as movies go. Now the Shannara series adaptation was just all-around dismal--the adaptation, acting, visualization, everything.
I agree with the Game of Thrones type approach to Stormlight. Stormlight definitely has a lot of visuals I'd love to see on screen if done well: the world-building, the shard blades, Shattered Plains battles, etc. Mistborn would totally have been the better 1st option. The Wax and Wayne would be awesome too but have a lot of spoilers to be firsts. Also, I think that the Sixth of Dusk story and Shadows for Silence would be great on screen, although they'd need some expanding and Legion would be a great mini-series.