Unfunnily ironically, Kalshi is one of, if not _the_ worst because it's not limited to one type of betting. It also seems to have lobbying power behind it that has shielded it from the kind of (laughably minimal) regulation that sports betting companies have to, because their lobbyists use weasel words like 'markets' in order to make it sound sophisticated rather than what it actually is: entirely degenerate.
Shameful.
Even worse because rigging professional sports is illegal, but similar manipulation in any other betting category is perfectly fine. Kalshi and co. are being used pretty openly by insiders. Trump's own family are major stakeholders and advisors of both Kalshi and Polymarket.