I think while Darnold could turn out to be very good in the long run, I think Mayfield is going to be the best of the bunch. He just has so much moxy.
As for Marrietta I just have never seen it in him. His highlight reels are always about his running; and that is not what you want to see when it's a QB. And when he runs, he gets dinged up and so rarely seems to be fully healthy.
Rosen has to prove his durability, but he is very smart and accurate. Kind of a Cutler, though, in his attitude and demeanor and that means he'll have a short shelf life with any one team.
Lamar Jackson is somewhere between RG3 and Michael Vick. The problem is that if you take away his running, you take away a big part of his game. But if he runs he won't last long. None do.