I recently wrote a quick review of every single 5E that came out so far for a Turkish hobby site, so I am actually quite prepared to talk about this! The books that I gave the lowest scores were:
Hoard of the Dragon Queen + Rise of Tiamat - This dead horse has been flogged a thousand times already, but they're just not good as adventures. It's railroad-y, the combat is imbalanced because the book came out before the monster math was finalised, and I think it just misses so many interesting story opportunities (did you know there's a section in Rise of Tiamat where you go to Thay... And it has nothing about what makes Thay interesting! No notable locales, no side quests, nothing. There is just a section on the social encounter with the diplomat who invited you - who receives you for audience once, then you get visited by Thayan wizards in dreams to get questioned, and that's it. Such a wasted opportunity to add something unique to the adventure).
D&D vs Rick and Morty Starter Set - It's just a subpar starter set that unsuccesfully tries to copy Rick and Morty's comedy style. The dungeon is bad, and they try to cover the fact that it's bad through
Lampshade Hanging.
Strixhaven: A Curriculum of Chaos - Not only did the Silvery Barbs spell make me lose confidence in WotC's design capabilities, I watched a friend try and make the mini-campaign in the set fun. No matter how hard he tried, he couldn't. Per his words, "The whole book is objects growing and size and attacking students". It fails to capture what makes Strixhaven fun as a Magic set, and just tries to make wacky a "College AU" version of D&D that is uninspired.
Dragons of Stormwreck Isle - Another Starter Set, and I think the worst one they made for 5E. It's laughably short, the story is forced as hell ("There is a dragon that
will intervene and save your arses and could solve every problem in the island...
But he's a pacifist!" This is Bad DMing 101!), and the D&D Cartoon tie-in is so badly utilised. They could've made the starter characters be the cartoon ones. They could've made it a movie tie-in and get synergy there. They did neither, and the cartoon characters are just there in the art... For some reason. Also, it doesn't feature the nice one-on-one play and sidekick rules Dragon of Icespire Peak had.
Spelljammer: Adventures in Space - Ship rules that can't allow any type of fun combat to happen, the whole Hadozee debacle, and the insanely short content expensively spread over three 64-page books. I just hope they learned their lesson for Planescape.
And a special mention goes to
Volo's Guide to Monsters, which was a great book for its time, but lost all value as an errata removed several paragraphs of cultural content (not just for mortal races, mind you, including bits about Mind Flayers as well), added nothing to replace it, and then all the stats in the book were upgraded in Monsters of the Multiverse. There is very little reason to buy the book now - and in any case, WotC doesn't allow you to buy it from any online marketplaces!