NFL.com does a great job of providing rating grades and analysis of all of the draft prospects. The grades are on a scale of 10 and Darnold was given a grade of 7.0. To put that in perspective, their legend shows that a grade between 7.00 and 7.49 classifies a player as "Pro Bowl to All-Pro Ability". His NFL comparison is Andrew Luck.
The grades for the other top QBs and their comparisons are as follows:
Rosen- 6.1, comp- Trent Green
Mayfield- 6.0, Doug Flutie
J.Allen- 6.0, Locker
L.Jackson- 5.9, M.Vick
Grades between 6.0 and 6.49 are noted as "Chance to become good NFL starter".
Comparing Darnold's grade of 7.0 to other position players, only 4 players (Barkley, Nelson, Chubb and Edmunds) were given a higher grade with Barkley the highest at 7.4
So, the NFL.com analysis supports the position that Darnold is probably the only QB that should be taken in the first few picks, and to me, the only one worth the Bills trading a king's ransom to move up for.