There are 6 quarterbacks who are being talked about as potential first rounders: Baker, Mason, Sam, Josh, Josh, and Lamar. Looking at each player's most productive college season (now in the 
By Draft Pick tab of my spreadsheet), here is how they compare to the other QBs who have been drafted in the first 2 rounds since 2009:
Player           Best Season
Baker Mayfield    2016
			Jameis Winston    2013
			Sam Bradford    2008
			Johnny Manziel    2013
			Marcus Mariota    2014
			Robert Griffin III    2011
			Andrew Luck    2010
Sam Darnold    2016
			Mason Rudolph    2017
			Cameron Newton    2010
			Tim Tebow    2008
			Matthew Stafford    2008
			Brandon Weeden    2011
			Mark Sanchez    2008
			Jared Goff    2015
			Teddy Bridgewater    2013
			Deshaun Watson    2016
Lamar Jackson    2017
			Geno Smith    2012
			Patrick Mahomes II    2016
			Andy Dalton    2010
			Ryan Tannehill    2010
			DeShone Kizer    2015
			Colin Kaepernick    2010
			Mitch Trubisky    2016
			Jimmy Clausen    2008
			Christian Ponder    2009
			Paxton Lynch    2015
			EJ Manuel    2012
			Blake Bortles    2013
Josh Rosen    2017
			Josh Allen    2016
			Brock Osweiler    2011
			Jake Locker    2009
			Derek Carr    2012
			Josh Freeman    2008
			Christian Hackenberg    2013
			Pat White    2008
			Blaine Gabbert    2009
College production is far from perfect as a predictor of NFL success, but at this stage this seems like a plausible tiering. 
PFF agrees with putting Mayfield on top. The biggest problem with this tiering is that I'd add another gap between the Joshes - I think Allen deserves to be farther back based on his awful 2017 season (which was significantly worse even than Gabbert's 2009), and shouldn't get drafted in the first 2 rounds.
		
 
After looking some more at the 6 QBs that everyone's talking about, here is how I'd rank them for the NFL draft:
Tier 1: Baker Mayfield, Sam Darnold
	Tier 2: Josh Rosen, Mason Rudolph, Lamar Jackson
	Tier 3: Josh Allen
(This is taking into account things like 
scouting reports and other people's rankings, in addition to my own 
numbers and analyses.)
Starting with 
Tier 1:
My numbers have 
QB1 Baker Mayfield with 2 of the top 3 college QB seasons of the past decade (Russell Wilson 2011 is still #1). 
PFF has him with similarly elite numbers, including in breakdowns like passing under pressure and difficult "NFL throws". [Edited to add:] 
Football Outsiders loves him too, and reports that he has elite numbers across various breakdowns in Sports Info Solutions charting. 
Pass velocity tracking at the Senior Bowl has him with good arm strength (though I wish they'd broken it down into more bins). Consensus first round pick. 
Scouting reports praise him for what I see as key QB traits like accuracy and progressing quickly through his reads.
The main negatives are his size (just a shade bigger than Drew Brees), inconsistent mechanics, concerns about Oklahoma's QB-friendly offense, and character questions. Those all matter somewhat, and the fact that most experts don't have him among their top 2 QBs is a negative, but it's not enough to knock him out of the #1 spot. Size: see Drew Brees. Mechanics: not clear if they're worse than Darnold's. QB-friendly offense: Mayfield's lead on the #2 quarterback this year was larger than the gap between Rosen and the average QB; he'd still have very impressive production even if you took a substantial discount for his situation. And Oklahoma doesn't seem to have off-the-charts levels of QB-friendliness; Landry Jones's production in Oklahoma was in the same ballpark as what Cody Kessler, Matt Barkley, and Brett Hundley did at USC and UCLA (and Trevor Knight & Blake Bell did significantly worse), and Mayfield's receiving corps has been pretty good but not close to Beckham+Landry level. Character: the main question is whether he puts in the work to be good at his job, and I haven't heard negatives about that.
QB2 Sam Darnold is the other obvious candidate for the #1 slot. He is neck-and-neck with Rosen for the top spot in most rankings, and he had great numbers in 2016. So he checks both the "reputation" and "production" boxes, and he has good size. But he doesn't check either box resoundingly. In expert rankings he is neck-and-neck with Rosen, and often ranked about 5th overall, rather than being an obvious #1. And his 2016 production was still well behind what Mayfield did in both 2016 and 2017, and his numbers slid to merely "above average" in 2017. So I have him slotted in behind Mayfield on the top tier.
Taking things out of order, let's jump to 
Tier 3:
QB6 Josh Allen had terrible 2017 season - my numbers have him ranked 86th out of the 100 QBs with the most attempts. In his favor his 2016 production was above average, so he has a better track record than Hackenberg, but his 2016 season still involved a 56% completion percentage and a worse-than-average sack rate. On the whole, it's a pretty bad track record. Evaluators tend to see him as a 1st round pick, but the 
strengths they talk about are mostly things like arm talent, having all the tools, protypical frame, can make all the throws. Nice traits to have if you're making a highlight video, but not what it takes to consistently drive your team down the field. If I try to force myself to be optimistic about Allen's future, I feel like I'd also have to tear into Wyoming's coaching staff who somehow managed to run such a horrible offense (105th in scoring) with a first round NFL talent at the helm; apparently they failed to surround him with talent, failed to design good plays to get guys open, and failed to coach him up to help Allen develop from a raw kid with a big arm into someone who was actually good at quarterbacking. But Craig Bohl seems to be pretty well-respected, and it seems more likely that Allen just isn't a first or second round talent. So I have Allen down here on tier 3; I'd be very unlikely to wind up taking him at his market value but I wouldn't give up on him entirely just yet because I'm far from perfect and this and evaluators might be seeing something that I'm missing.
Back to 
Tier 2:
QB3 Josh Rosen is neck-and-neck with Darnold for the top QB spot, in the expert rankings I've seen. I have him at the top of tier 2 in deference to those rankings. But his track record is surprisingly unremarkable, given his reputation, and is closer to Allen's than to Darnold's. He was an above average quarterback in each of his three seasons, with production (in my numbers) similar to Allen's peak year in 2016 and a bit behind Darnold's down year in 2017. In Rosen's favor, his 
scouting report praises him for things like his footwork in the pocket and his accuracy, which are the sorts of skills that seem important at the NFL level. And 
PFF's grading system seems to like his production a bit more than my numbers do; they have him as the #5 pick (and 3rd QB) in their 
latest mock. On the whole, Rosen checks the "reputation" box but he doesn't check the "production" box well enough to be on tier 1, but his production has been decent enough to still lead this tier. One more negative which is more relevant for fantasy than for NFL (though it matters for NFL too): Rosen has negative rushing yards (with sacks counting as rushes), with just one 15+ yard run during his college career. That will put him in a fpt hole relative to most QBs under age 33, with Carr & Goff as exceptions, and Football Outsiders has also found that negative rushing yards is a predictor of lack of NFL success.
QB4 Mason Rudolph checks the production box as well as Darnold, but he is generally seen as a rd 2 prospect. And OK St. has been pretty QB-friendly - probably even moreso than OK - just look at 
J.W. Walsh's career stats, plus Washington & Ateman are a nice WR duo. I'll be relatively optimistic about Rudolph if he's a top 40 pick, but he could be another Bryce Petty.
QB5 Lamar Jackson has obvious limitations as a passer but he's electric as a runner and seems to have improved this year as a passer. His overall production rating this year was similar to what Watson & Mahomes did in 2016; if we just look at that bottom-line number and ignore how he got it then he slots in right behind Rudolph. Skill as a passer seems more important than skill as a runner, which is a reason to discount Jackson a bit relative to his bottom-line production rating, but Rudolph is also getting discounted some due to his offense. And Watson's early success in Houston seems like a good sign for Jackson's NFL potential. It'll help if Jackson gets a coaching staff that will play to his strengths and a good go-up-and-get-it WR1; from what I've seen he likes to throw sideline passes that give his receiver a chance to make a play on the ball. For fantasy rankings I'd move Jackson up to QB3, and to tier 1.5, because of the fantasy value of rushing production.