I think Darnold has the highest ceiling (and unlike Allen, I believe that it's attainable). I think he'll probably be the same high variance type of QB as he's been in college, he just needs to get rid of the really stupid errors from his game. He'll throw picks because he's hyper-aggressive but I can live with that. Daft fumbles? Not so much.
I haven't focused much time on Sam because I'm certain that he's going #1. He's my QB3 after Rosen & Baker but I'll gladly take any of those three.