The guy I'd worry about busting the most is Blake Bortles. I really don't understand the hype, sure you see a 6'5" 235lbs prototype QB who put up decent numbers at UCF passing and somehow became a scrambler near the end of the year but some of the comparisons "professional" analysts are making seem to be absurd to me. My favorite comparison is to Ben Roethlisberger, Bortles has half the arm Ben has and most of those "scrambles" he got away with in college would end up with him getting speared by Suh.
The best QB in the draft will be Teddy Bridgewater followed by Manziel if he gets in the right system behind a good OL and a talented WR. I hated Manziel at first when projecting him a couple months back but I believe it's less dependent on talent, which I think he has tons, and more dependent on coaching and teammate talent on offense. He's going to be a hard bust if you expect him to take control of an offense like Cam but if you let him do what he's best at, you'll get a playmaker. I think Bridgewater has been underrated due to a bad pro day. I look at his tape and to me he goes through progressions better than any other QB in the draft, fragility is my biggest worry with him.
Overall I think it matters less about which QB and more about the situation.