Personal theory time: I believe that there are only a handful of guys who can look like a "legit NFL QB" in any situation. Most guys are at least somewhat dependent on the "context" - having the right coach to unlock their potential, the right system, the right guys around them, etc. Among the guys currently playing, I think you've basically got Brady, Roethlisberger, Brees, Rodgers, Newton, Wilson as the guys who are clearly going to stand out as star-level quarterbacks regardless of what's going on around them (plus a guy like Andrew Luck, who had carried the Colts through his first three seasons before struggling with injuries last year, but everyone seems to be unanimous that he's a stud and that last year was a fluke).
Beyond that, there are plenty of other guys who can range from "Man, he looks pretty good!" to "Why are we starting that scrub?" depending on what's going on around them. There are some other guys who I really like (Philip Rivers, Alex Smith, Andy Dalton) that are clearly "capable" NFL quarterbacks but are at largely dependent on having some things break right around them. Dalton in particular gets a lot of hate that I don't understand (his first four seasons are, believe it or not, pretty favorably comparable to Peyton Manning's first four seasons in the NFL). I also happen to think Eli Manning is a bona fide star and would be delighted for my team to line him up on Sundays, but there are tons of fans who think Eli sucks and only starts because his name is Manning. Beyond that you've also got guys like Derek Carr, Blake Bortles, Matt Stafford, even Jay Cutler and Ryan Tannehill that have shown they can play at a passable NFL level when surrounded by competent coaching and talent.
Finally, one of my favorite examples any time a discussion like this comes up is Rich Gannon. Drafted in 1987, started for parts of five seasons in Minnesota, Washington, and Kansas City, never doing anything more than pure-journeyman stuff (I mean, Gannon's career stats from 1987-1998 look basically like Shaun Hill's career trajectory, right?) before taking over the Oakland job in 1999 at age 34...and promptly making four straight Pro Bowls, two All-Pro teams and a Super Bowl. Imagine if Ryan Fitzpatrick took the Jets to three straight division titles and a Super Bowl in the next three seasons. That's pretty much what Gannon did starting at age 34 after an otherwise pedestrian career. What changed? Well suddenly Gannon had an offense with Tim Brown, Jerry Rice, Charlie Garner (soooo underrated!) - you get the picture. Context is a huge factor for the "good-enough" guys.