The Supreme Court’s Citizens United ruling is a consummately extreme and ideological document, the product of the Federalist Society mindset through and through. So if you’re trying to elevate one specific part of the ruling as especially disconnected, you’ve got a lot to work with. In fact, it’s one of those questions for which there is no “wrong” answer.
If I had to choose just a single piece of the decision’s underlying argument as the most unhinged, though, I’d go with Justice Anthony Kennedy’s assertion that “independent expenditures do not lead to, or create the appearance of, quid pro quo corruption.” I’ve written about this previously, so I’ll try not to belabor the point. But the key thing here is that Kennedy isn’t just saying super PACs don’t corrupt. He’s saying they don’t even create the appearance of corruption.