well my personal picks are:
A song of ice and fire, look even though I'm straight, I know Peter Dinklage is the hottest man under 5 feet (I don't think anyone can disagree), and that plus the performances of Joffery, Ned and Robert Baratheon are absolutely amazing and add a ton to book one for me. But there's not really a point in adapting A song of ice and fire because whatever the show adds in the novelty of visual effects it takes away from the general scale and it continues this pattern until there wasn't any good source material left when they had the budget for the real battle of Blackwater, with bodies falling from the sky and shit. There's just also the fact that the fact a lot of the characters are aged up takes away from the beliveability of their decisions. In the original book, Robb Stark starts at 15 which explains his actions better than the show right away, since some of his decisions are based off his own naivitey and the fact that he's kind of a horny little shit and Jon Snow is only 14. A lot of the conflict from the books is based off the fact that these guys where sent to living hells or forced to take a position in court even though they were playing with wooden swords at the beginning of those same books and the most disturbing scenes in the entire series are kind of dumbed down in the show. Other than that you've probably heard every other criticism towards GoT.
Harry Potter: Chris Columbus did a great disservice to Hermione and made her a flawless Mary Sue for no reason. Seriously, her flaws where based in anxiety and every smartass thing she does in based in an earnest interest and love for the world she lives in, in the movies she comes across as an annoying brat that's never been told that sometimes people just don't want to listen so Columbus took it all out and let the other directors follow in his footsteps, even if the movies were still "good" that's only because the source material was great.
Percy Jackson Another fuck up by Chris Columbus. Color me impressed. He's pretty much the reason why any of you guys may have not touched one of the few book series met for the same demographic as Harry Potter and is probably just as good. Seriously screw Chris, man. Percy Jackson really deserved better. Still definitely worth the read even if it's not as well carried over to an adult audience.
The Walking Dead It's been a shadow of it's former self since Season 2. The comic still good though even if it's almost as repetitive as the show.
Any way other than that there's only one book series I'm happy will probably never be adapted, The Saga of Recluce it's really damned good, too slow but really damn good, it's definitely worth the read no matter who you are or what you're looking for as long as you don't care if it's a tad slow. What are some things you wish where never adapted?