Stephenson, no doubt. Langer was good, but when you discuss the best center of all time, the discussion generally narrows down to Mike Webster vs. Dwight Stephenson.
You've also got to consider the idea that the Dolphins running game in the era Jim Langer played was not the product of any one individual. Langer is in the Hall of Fame, and the two Guards next to him respectively are a Hall of Famer and a perennial HOF nominee, not to mention that they were run blocking for a HOF fullback, and pass protecting for a HOF QB who was throwing to a HOF WR.
I think there is a very strong argument that Miami's success in the early 70's resulted in players who were very good rather than great making it into the HOF, especially when you've got so many of the players named in compliamentary positions. Maybe Langer was a very good Center who looked like a great Center because of Larry Little or Bob Kuechenberg playing next to him, or Larry Csonka running behind him, or any combination of that?
That's also why I dont think we'll ever see Kuechenberg in the HOF.