What exactly "reasonable laws" and "excesses of free speech" are is not an easy topic in itself and lots of reasonable trade-offs here, but I would start with threats of violence. This includes not just "I will punch you in the face" which is already illegal in the US, but also general statements encouraging violence which are typically not. For example "Hitler did nothing wrong, we need to clear the lands of Jews", "the NYT building should be bombed with all the journalists still in it", (Ann Coulter on several occasions), or celebrating the deaths of gay people (Rush Limbaugh). "Haha, only serious"? There is a very direct and straight line between all of this and Trump, and I don't think we would have had Trump today if it wasn't from four decades of non-stop high-aggression vitriol stuff from people like Coulter, Limbaugh, etc. etc.
If you want to have a free "marketplace of ideas" then you need to have some sort of baseline. This baseline doesn't need to be very high, but I would say that "I encourage your murder and everyone like you" is below it.