Call me a child of the 80s, but...
Hobgoblin. Used to be awesome. These days he's a bit of a joke.
Alpha Flight. Marvel's biggest punching bags. Not even John Byrne will stick up for them. Bill Mantlo made it his mission in life to destroy the original team. But the Alpha Flight format - each character gets an individual showcase, and when they finally unite as a team it's a big deal - was used to create the MCU. A particular shoutout to Aurora, the most complex and interesting member, who is now basically a supporting character for Northstar.
Monica Rambeau. One of the strongest new-character introductions in years, and an exemplary leader of the Avengers. First she gets ejected from the Avengers by Doctor Druid, of all people. Then a succession of writers keep giving her latest identity to another character. I like to think this didn't have racist intentions, but she was marginalized more and more over time, and it left a bad taste in my mouth.
Richard Rider. From one of the most critically acclaimed and beloved stories in Marvel history, of which he was undisputed star, to being written off and replaced by a bungling kid. I don't understand how he could have gone from such a big deal to being irrelevant so quickly.
Scarlet Witch and Vision. I know they are a big deal now (especially in the void of other MCU content). But for awhile, writers couldn't stop portraying Vision as a toaster, and Wanda as a raving loon. Everyone seemed to drink Bendis' Kool-aid that Vision was essentially a walking adult toy for Wanda, which insultingly portrayed Wanda as even weirder/more crazy and trivialized Vision, who was, for a decade or more, the heart and soul of the Avengers.
Yeah, the Nova one was rough.
It was very much like we just need to replace with whatever and it will be fine. It was not fine and they treated a very cool character very badly indeed.
The others I agree with but loved the Nova series and his role in Annihilation. Since then has been a bit of a mess.