If we are talking all time? The Packers hands down.
If we are just talking about the Super Bowl era (i.e. 1967-present) the Steelers barely edge out the Cowboys and the 49ers.
For recent times, say the last 15 years, its a little different
New England Patriots: (3-2)
Denver Broncos: (2-0)
New York Giants: (2-1)
Pittsburgh Steelers: (2-1)
Indianapolis Colts: (1-1)
Green Bay Packers: (1-1)
St. Louis Rams: (1-0)
New Orleans Saints: (1-0)
Tampa Bay Buccaneers: (1-0)
Baltimore Ravens: (1-0)
Arizona Cardinals: (0-1)
Chicago Bears: (0-1)
Seattle Seahawks: (0-1)
Philadelphia Eagles: (0-1)
Carolina Panthers: (0-1)
Oakland Raiders: (0-1)
Tennessee Titans: (0-1)
Atlanta Falcons: (0-1)
Out of recent history, I'd say the Patriots are the greatest team, and the Giants and Steelers follow them up. The 2 Denver wins happened over a decade ago. So they deserve some recognition, I just don't know if I would group them in "this-era" of greats.