You're opening a can of worms.
Some say stock. Some say K&N.
I have an Airaid, I got it years ago, I used to get 19 mpg's when I got my truck, now I get about 20-21. It may just be because that when I hear the whistle, I ease up on the gas.
Honestly, do what you want, but you're going to get a lot of differing opinions