The thing about the internet is people can crosspost. So if the worst kind of person posted on Twitter, there's nothing stopping them from creating another account and posting on Reddit, or any other social media platform for that matter.
Based on this criteria, the worst would be Facebook. It has the most users, so of course it will have the accounts of the worst people. Though if you could expand social media to include Wikipedia-type sites, then the highest concentration of the worst people would be Rational Wiki (don't let its name fool you).
However, I would rate social media platforms based on their ability to provide me tools to filter and mute the worst kinds of people. I don't like platforms that do its own filtering without my permission; like Twitter shadowbans posts from users, and I would rather be fully responsible in determining at any given moment what users I want removed from my timeline. Because of this, the only good social media I like nowadays is 2ch and 2chan; I have nearly full control of what I see, and neither the moderators or algorithm is pretending to be some overbearing parent.