Big Ben is the most physically imposing quarterback in the NFL. He might never win an MVP or be a cover-boy for GQ magazine, but every year he plays, the Pittsburgh Steelers are in the championship race.
In addition, Roethlisberger, unlike Manning and Brady, has won Super Bowls without a world class offensive line or the Hall of Fame-caliber wide receivers that Manning and Brady play pitch and catch with.
Roethlisberger has made a career out of extending plays with his legs and brute force. Manning and Brady are pure pocket passers, who are much less effective or mediocre when they their pockets collapse.
Roethlisberger, on the other hand, wins with or without a dominant offensive line or all-time great wide receivers, and if he retires with three or four Super Bowl championships, which is totally possible, he will cement his legacy as the greatest quarterback of his era.
If the Pittsburgh Steelers win the Super Bowl this year, which I believe they will, Big Ben has the most legitimate claim as the best quarterback of this generation.