Death Guard for Iron Hands.
The Death Guard were dicked by Typhon.
The Iron Hands are completely misguided madmen who have totally missed the point in being Astartes. That's the point in them of course and it's cool, but in terms of which swap would be best for the Imperium, that's the swap I would make. They see themselves as superior to the humans they were created to save, and usually kill whoever they 'save' afterwards. They win fights, but are the worst chapter by far in terms of how they deal with others. I imagine the only reason they haven't been declared traitors is their close ties to the mechanicus. They're probably worse than a lot of actual renegades.