It's a stupid question, because  there are thousands of guys who played quarterback in the NFL.  Most don't register on the radar screen, because they only played a game or two.  But I guarantee, there are literally thousands of quarterbacks in the history of the NFL who were worse than Blaine Gabbert. 

 

See, the problem with "who's the worst ever" type of questions is, talent levels are spread like a pyramid.  The best players are at the top, with very few other players.  So you can actually ask the question, "who was the best."   But the worst players are at the bottom of the pyramid, with thousands and thousands of other players.   So you cannot ask "who was the worst" unless you eliminate most of them with a qualification, such as, "who was the worst  quarterback who was selected in the top 10," or who was the worst quarterback who started at least 20 games."  

 

It's just a poorly defined question, unanswerable as it is. 

---