The top quarterback in Vikings franchise history is Fran Tarkenton. Nobody would refute that. After him, a debate can be had between a multitude of players. Although numbers got inflated because the game has changed, two guys are fighting for the spot behind Tarkenton on the franchise leaderboards: Tommy Kramer and Kirk Cousins.
Ex-Vikings QB Advises Franchise to Reinforce 1 Area of the Team

While Cousins has spent exactly half of his 12-year career in Minnesota, Kramer only left the Vikings for his final season in 1990 when he joined New Orleans, where he played in only one game, ironically against the franchise he was a part of for 13 years.