For any of these guys, I think if you get him with a Kubiak, Shanahan, Mike Martz, Josh McDaniels, Mike McCoy... or hopefully Bill O"Brien I think they will do very well..... good Schaub would be their floor..... jmo. It's up to them to bring the playmaking ability with them.
I like the Mettenberger, Murray, & McCarrons better because watching them I see them hang in the pocket & work their system more than the other guys. Even Bridgewater. These guys pump fake, throw back shoulder, look off safeties, & take hits in the pocket as they throw a fairly accurate ball.
Murray & McCarron, I think are the most NFL ready. May not be the most athletic, don't have a lot of wow. Mettenberger & Bridgewater are probably next on my list. Then Fales & Bortles.
Manziel is totally different..... on a scale all his own, more like Vick, Newton, & Young. Mostly wow. He doesn't have the arm those three had & he doesn't have the size of Newton & Young, or the speed of Vick (or Newton).