OMG. The memories, they hurt!
I submit Avengers vol.3 #71 by Geoff Johns. It should have gotten him and the editor Tom Brevoort fired. You can't even see all of it in the TPB version because they took a page or two out. It isn't that they dwell on Hank and Jan, again (see more on that below, that wasn't the worst here) it's that in the Avengers, not the Ultimates, the Avengers, they have a size changing sex scene with Hank and Jan and follow it up with Whirlwind beating up and murdering a prostitute he had dress up like Jan. Inexcusable and unforgivable.
Some other nominees from the Avengers in order:
Wanda raping Simon in Avengers West Coast 56-57 is in second only because it's one thing instead of two and the art isn't as graphic. Byrne and editor Howard Mackie should have been fired.
Assuming Michelinie's story of what happened is true, Avengers 200 was a last minute rewrite. The story they basically wanted to use got ninja'd by a few months and who knows what they were thinking when they had Kang's son use mind control to rape Carol Danvers so that she would conceive and bear him as a child. And at the end the other Avengers just let them go off together (no mention of possible mind control or anything). Would it be better if the original with the supreme intelligence artificially inseminating her had happened? I really like Chris Claremont's Avengers Annual 10 that addresses this - but I've got nothing to say to against anyone who finds what she went through in that one awful too and it deserving a nomination.
The slap (whether it was an artist mistake or intended) in Avengers 213 was awful, but the dozen or so issues after that worked the storyline out and numerous authors have dealt seriously with Hank Pym's mental health and spouse abuse. The Wasp series about Nadia a few years back did a spectacular job. So 213 isn't my nominee - it could have been a tragedy that led to a continuing story of the road back from some betrayals and the lifelong struggle against mental illness. On the other hand, numerous authors have made a joke of spousal abuse and/or mental health (ignoring both the issues after 213 and the work of the other writers who followed in that spirit). All the writers who did that were at best having naughty word weeks, at worst are naughty word.
And now some lesser ones.
Byrne destroying Wanda and the Vision more generally in West Coast Avengers.
Anything putting the Sentry into the 616. I hates it with the power of a 1,000,000,000 burning suns. Invalidating the history Marvel comics, lazy powers, and insipid writing.
Busiek's Kang Dynasty. Lazy world.threat and trashing where vast parts of the surface are mangled by an insane secret defense grid, a country becomes nuclear zombies (granted Thor is awesome in it), Washington DC is nuked, all the worlds heroes are locked up in concentration camps ... and the repressions are gone in two issues (compare to, say, 9/11 in the real world or even the snap in the MCU). Most overrated Avengers story of all time - and it isn't close.
A few Avengers issues by Bendis where he has clearly decided that part of his story is insulting all the long time fans. He had some good stuff, but he was also an insane child here and there.