Spoilers! For those of you who have yet to complete all the campaigns, go do so first then come back.  

Jake and Sherry's. Not necessarily because of the actual campaign, but the character themselves. Jake is just fucking awesome, Sherry being teamed up with him was strange at first... but turned out to be perfect.

Gameplay wise though, It has to be Leon's. I loved the part where you were fending off zombies with a group of survivors.

I disliked Leon's the least.

Jake and Sherry , pretty easily .

They were the only characters I somewhat cared about , and it's the only campaign that didn't have an awful boss fight in ... Plus that helicopter fight with the Nemesis look-alike was the MOST AMAZINGLY DUMB AND AWESOME PART IN THE GAME ! :p

Easily Jake and Sherry.

I liked Ada's the best because it was the last one...

Jake and Sherry. I really enjoyed the character interaction between the two. Chris' was second - much like you, DarthOrange, I really enjoyed the snake boss battle.

Chris & Piers by miles; don't like Leon never will but Jake/Sherry was decent and Ada Wong would've been good if I didn't know the entire plot before I got to her.

Ada Wong's campaign was my favorite, for no real actual reason other than it kinda felt alright while playing it...

That said, the best moment in this game is hands down the INVISIBLE SNAKE battle!!!

1. Ada's campaign. Aside from the fact that it is Ada, it reminded me a lot of Separate Ways and RE4 in general. The combat/puzzle flow was the closest 6 came to 4. Plus it had the least B.S. moments out of all four.

My favorite was Jakes campaign I think. Just seemed to have the most variety and it had the most interesting story line.

Leons campaign I felt was alright but the overly long final boss fight took a little out of it so it's probably my least favorite one. Chris' campaign opened really terribly. The entire East-European chapter was just super boring. When you got into China it got really fun with the fight against the invisible snake being the highpoint probably. Adas campaign felt really.. boring? I mean it was cool to play some of the areas you had played with other characters but there was barely anything new. Also the Simmons fight was still way too long.