Best "quarterback" is always going to be a subjective argument. Superbowl rings, post season accomplishments, longevity, surrounding talent, etc. are always going to be variables.
Dan Marino was the best pure PASSER of all time, and you won't find many that will dispute that... not even former NFL quarterbacks who rank right up there in his neighborhood statistically. That also includes Hall of Fame caliber defenders that had to face him in his prime...
There's just not many quarterbacks that have ever played this game that could've accomplished what Marino was able to accomplish for as long as he did, with the teams he played on. Particularly in an era where the rules didn't allow for receivers to just run free all over the place, and brutalize quarterbacks the way they could 20 years ago. You can probably count 'em on one finger...
Dan Fouts and Warren Moon are the only two guys I've ever seen that could even be kept in Marino's company when it comes to the pure ability to pass the football...