I'm pretty low on the QBs that came out of school this year, especially Josh Allen. He's going to be an even bigger project that Lamar Jackson, IMO.
I think Rosen ends up having the best career out of all of them, followed by Jackson, because the other QBs went to franchises that are far more likely to screw the pooch. That and I think Josh Allen is a horrid QB.
Rosen is going to go to a team that has some nice pieces offensively, namely Fitz and David Johnson, and has shown to be relatively patient when it comes to the QB position. They don't throw QB after QB into the fire out of desperation like the Browns, they're actually alright with sticking with someone like Drew Stanton or Sam Bradford and have the kid develop without the pressure of producing in year 1. With Rosen's mechanics and ability, I think he puts on a little weight and matures enough to become an average starter in the league.
Likewise for Jackson, he's going to get the opportunity to sit behind Flacco and work on his mechanics, understanding the pro game and his pocket passing. He won't be pressed to produce from the get-go, but even if he was, he would probably be one of if not the most exciting rookie prospects to watch. Don't forget he was a Heisman winner and has played in a good conference.
I see the Browns doing the regular Browns thing. They'll start Tyrod Taylor for a few games, which they'll lose, but Taylor plays alright, and then the fan base will start pining for Mayfield because "OMG 1ST OVERALL PICK." So senseless-Hue will throw Mayfield into the fire, and he'll look alright for a couple games, before posting an awful 1/4 TD/INT game where they lose badly, and the dude will never recover his confidence, at least for the rest of the season. He'll get another shot the following year, with a new coaching regime, but the Browns curse continues after that and he makes a career out of being a journeyman back-up elsewhere in the league.
Darnold is more up in the air. I think going to a New York team won't be so bad for the kid out of USC, but I would have much rather preferred to go to the G-men than the Jets. At least NYG has had SOME FORM of success this century. Darnold will have the opportunity to win the starting position from McCown, but I expect McCown to hold it down for the majority of the season -- barring a big dropoff from his relatively average 2017 performance. I do see McCown helping Darnold grow and mature early on, as McCown is a consummate pro and has learned that being even a journeyman in the NFL takes a ton of hard work and perseverance.