The usuals -
Brett/Casey - The show is just embarrassing itself at this point, trying to force this devoid of chemistry mess (I miss when chemistry was a requirement for couples on this show, they seem to have given up
). It's so forced it's embarrassing to watch and you can tell the actors know that. Casey went down the drain as a character a while ago now (Mouch and Gallo are still waiting for their apologies, btw), and their solidifying Brett as Derek's "Gabby placeholder" is just taking her with him. Words cannot express how much better she deserves than this utterly offensive mess. The days when she was her own character are far behind her.
Jay/Erin - He was basically made a footnote in her story. The entire relationship was about her and what she could get from him. Completely toxic for him. She never communicated, barely ever acknowledged he even had his own issues, which played a big part in why his PTSD was never addressed until after she ghosted him (and as awful as that is, he's so lucky she did).
Phoebe/Cole - Literal abuse. He wanted to annihilate her family. Brought out the worst in Phoebe, and in some ways, Piper. Almost cost her another sister after losing Prue all but destroyed her.
Brooke/Lucas - Completely toxic for each other. Brought out the absolute worst in each other, and neither of them cared to actually get to know the other. Brooke had to manipulate the situation to even get him to look at her in the first place.
Rory/Jess - Completely toxic. None of Rory's boyfriends were great, but Jess was by far the worst. He had absolutely no respect for her or her boundaries (as shown by the sexual assault), and he was abusive and condescending. The way he treated her was absolutely abhorrent. Beyond cruel. He didn't love her, he wanted to own her, to control her, and when she wasn't willing to play along, he berated and punished her for it via disappearing acts. Whenever he came back, he immediately started trying to control her again.
Emily/Alison - Glad the spin-off ended this toxic disaster (MK's fanfic tweets aren't canon, because it didn't happen on screen). Alison is a sociopath, the show trying to pretend she wasn't was one of the worst mistakes it ever made (and it made plenty), and it would be even if it made sense, which it didn't - a sociopath cannot just stop being a sociopath. Idk why they thought promoting sociopathic abuse as true love was ever a good idea...
Kara/Mon El - Abusive mess that supported the idea that it's a woman's job to fix a toxic, damaging man. Sidelined Kara in her own show. The fact that the writers didn't see an issue with essentially replacing Kara's relatively healthy relationship with a genuine, kind black man with an abusive relationship with a slave-owning misogynist is concerning at best.
JJ/Reid - So glad they never followed through with that retcon. What they did do made absolutely no sense.
Buck/Abby - The way she treated him was pretty appalling. She's entitled to a midlife crisis, but she used him and left him hanging when she wasn't coming back, eventually ghosting him.
Buck/Eddie - There's never been anything romantic between them. Imo, it's never even been hinted at. Maybe it could work someday but as things stand now, it wouldn't make any sense. It would basically be the JJ/Reid retcon all over again. If they want to make it happen they have a lot of work to do.
Georgie/Izzie - Their friendship was beautiful and as with all beautiful things on GA, Shonda destroyed it. There was no reason for this mess to happen. Ever. There was no romantic chemistry, it was embarrassing to watch, and destroyed an amazing part of the show.
Meredith/Derek - I used to love them when I was younger but it basically started as romanticised sexual harassment.
Tom/Ann - Idk why this happened, but it shouldn't have.
Amber/Bray - Nope. She willingly believed a lie told by someone she knew hated her and used it as an excuse to abandon her found family, which was apparently his fault?
Ellie/Luke - Beyond pointless, and kinda ruined Ellie's character.
Cloe/Ved - They were both way too immature.
But also -
Amber/Jay
Trudy/Jay
Ebony/Jay
Haley/Dylan