If you measure greatness by a coach's innovation and creativity, then Tom Landry, Paul Brown and Bill Walsh would be good choices, as the league was filled with their disciples. For their contribution to the league, George Halas' forty seasons of coaching stands alone. For success (titles), Chuck Noll and Vince Lombardi are in a class by themselves. From the AFL, Hank Stram and Sid Gillman should be part of the discussion for their undisputed records as leaders. Dick Vermeil and Don Shula took different teams to the SB, a very rare feat.
I think you mean Landry "only" won two Super Bowls, although this seems a dubious argument. Landry's Cowboys were prevented from winning four more SB's by the Packers and Steelers, two of the greatest teams of all time. Twenty straight winning season, with 18 playoff appearances is a stunning achievement for a coach, maintaining a level of excellence with continually new players.