Marino was one of the greatest QB's of all time, certainly one of the greatest I've seen. Top 5. Beyond that, it's almost impossible to differentiate those guys and ends up being personal (fan) preferences. I tend to not like the term greatest "passer", since there is a bit of a back-handed complement attached (i.e. but not a great QB). He carried bad/mediocre teams, he helped win games Miami never should have won and kept Miami in games they never should of had a chance in. He dominated his era, and this was back when the rules were not nearly as slanted towards the passing game. His pocket mobility/presence is probably unmatched (I have not seen any QB do what he did in the pocket, in terms of sliding and stepping to avoid the rush) and his release is also practically unmatched as well.
He is #1 all time in 4th quarter comebacks. I know some aren't crazy about this stat, but for me it shows LEADERSHIP - which is a lot of what being a great QB is about. Put the ball in his hands with the game on the line in the 4th quarter. That opportunity never presented itself much in the playoffs, due to the team not really ever being great.
I dredge up these stats occasionally, just to contrast the effect of TEAM on QB effectiveness. In Marino's 10 playoff losses, Miami gave up an average of ~210 yards rushing and ~34 points a game. 7 of those 10 games they gave up >200yards and >30points. Montana had 2 playoffs games where his team gave up >200yards rushing and >30points (both crushing losses). You basically can't win games against good teams giving up 200yds rushing and 30+points.
Montana's team gave up 16 points or less in 50% of his playoff games. ~33% of the time, they gave up 10 points or less. For Marino, it's ~19% (16 points or less) and ~10% (10 points or less). Those differences are pretty staggering, IMO.