whats the best engine for both performance and mileage??

Everyone's definition of "best" is different. As you can read. Me personally, a 360, built mild, with 3.55s and OD had better get 18-20 in an A body. There are larger engines, with no OD, in heavier cars, with less gear getting 17. So I'm confident it can be done assuming your definition of "best" is 300rwp N/A on pump gas and somewhere around 20-24mpg.
I'll also agre that Mopar's engines for the day were great, for today are pitiful, and that includes the "(cough cough) Hemi". Take a look under a Ford's hood if you want to see a really modern V8, and what they are capable of doing. You don't shut off cylinders to get mileage, you make the engine more efficient. That being said, I am a purist in the belief that if it's a Mopar, it should have a Mopar engine. But- that's way of topic and doesn't add to the discussion...lol.