Leon's campaign was actually more old school RE in a good way. (zombies, dark atmosphere etc). I think it is the best one in the game. Chris's campaign was more action oriented. It seems they wanted to make in more RE5 like. I give it the third place Jake's campaign is like a beta. Trying something new. Martial arts are too unfinished and stealth aspect in his chapter is not so enjoyable like in RE3 Nemesis for example. Fourth place from me. Also Jake's character is quite cliche and weak IMHO. Ada's campaign is the shortest one but it is quite good in terms of the gameplay, action and storywise. Also you are free to use stealth or just go gung'ho. That's the second one from me.
Leon's because the main story of the game was the outbreak, which Leon was in the middle of. Also it was nice to have that horror feel in Leon's. Gameplay for Leon's was good, some cool enemies, some weird ones, interesting scenarios, challanging.
If i had to pick a 2nd it would be Jakes. The gameplay was different, but it was still fun. It was cool that you were mostly dependant on yourself. The Ustanak is after you, not sherry. You have to sneak by yourself. Some of chapter 3 you were by yourself. Story wise, it was nice to see his relationship with Sherry progress. In the beginning he didn't care about Sherry, then after the plane crash in chapter 2 he was worried she might not make it. Also how he handles being the son of Wesker.
Chris's would be numero tres. Not that his was bad. Chris's really surprised me. Yeah the first 2 chapters were heavily action, it became a little darker in the 4th and 5th chapter. You were low on ammo throughout the whole campaign, and i actually beat the final boss with my last acid round, which was my last ammo in any gun. Scared me when he grabs piers, because i thought you had to shoot him, which i was unable to do. Chris's story reminded me of 1, how he found all his friends dead in the mansion. Chris has always had that tragic life.