This is a good question.
Darrelle Revis is an amazing player and probably the best cornerback of his generation. I'd say he is one of the top 10 CBs to play the game.
Calvin Johnson is far and away the best receiver of his generation and probably behind only Jerry Rice (and possibly Randy Moss) as the most dominant player at his position of all-time.
I'll take Johnson though I am thrilled to have Revis.
Meanwhile, I would take a 100% healthy Rob Gronkowski over either player. His blocking skills combined with his receiving/red zone abilities make him the best non-quarterback in the NFL. If he can just stay healthy, we are all set with game-changing offensive superstars who completely transform the offense.