If you want the story to be at its full potential Jakes first, it introduces the characters and gives a decent background on everybody. Chris second, it has a good story and you get some surprise twist in this campaign, and playing leon before chris ruins some story aspects from chris. Leons third, as it clears up the main story, and goes off of some of the stuff you find out in jake and chris campaign. Playing leons before those two kind of ruins the surprises in the game. Adas last, as it was meant to be. It clears up loose ends and has some surprises as well. You'll figure out why some strange things happened in other campaigns if you play hers, which is kind of a treat if your into uncovering more details as you go along.
IMO campaigns are Leon=Ada>jake>chris
(PSN:zombiabsol3) RE6 and PASBR <3 Waiting for: Remember Me, The last of Us, and Pokemon X.