According to the Pro Football Hall of Fame, there are 5 Vikings in the Hall "who made the major part of their primary contribution for" the Vikings: Page, Eller, Tarkenton, Yary, and Krause. There are a lot of great Vikings not in the HOF right now--Mick Tinglehoff and Jim Marshall probably should be HOFers, Cris Carter will be, Randy Moss should be, and other players such as Chuck Foreman made significant contributions to the team but aren't really HOF material. And despite never winning a title, the Vikings have a very rich history, playing in 4 Super Bowls winning 16 division titles.
So who is the greatest Viking ever?
For my money, it's Alan Page. Page was the first defensive player to win NFL MVP in 1971. He was the anchor of the Purple People Eaters. He played in all four Super Bowls. His quickness allowed him to do a lot of things that we'll never see defensive tackles do anymore. He was consistently the best defensive player on a team that more often than not relied on defense to win games.
I rank him ahead of Tarkenton because he played in all four Super Bowls, and because he was a more dominant DT than Tarkenton was a QB.
Tarkenton is wonderful. He retired the all-time leader in completions, attempts, passing yards, and passing TDs, he was the second Viking to win MVP (1975, and unless I'm mistaken, we're still waiting for the third), and his style of play was unique. But the Vikings first achieved success with Page as their best player, not Tarkenton.
Alan Page is the signature player in the Vikings' history.
And until they win a Super Bowl, I won't rank any other player ahead of him.