All 4 albums represent their most experimental period.
The 3 periods (early, middle, late Beatles) shows how music evolved from a naive, somewhat childish approach to music (up to Rubber Soul) over an experimental, "growing up" / teenagerish, maturing period (ending with Magical Mystery Tour) to adult, modern rock.
Pepper is the pinnancle of the experimental period showing maturation on many levels (production, composition (A Day in the Life), concept (or attempt at)). And then it is representative of a cultural phenomenon that was flower power, Summer of Love.
Pepper for me. But I like the salt of the other albums.(I guess Magical Mystery Tour is the Chili in comparison - too strong for my taste and maybe they went a little over the top with Strawberry Fields Forever - from a production point of view, which wasn't realistic in the long run. Well they did abandon that approach future recordings but I appreciate the attempt.