What pronoun changes its form least as its case changes?

The personal pronouns you and it do not change form from subjective to objective.

Examples:

You may have some cake. It is very good. (subjective)

I made a cake for you. I hope you like it.(objective)

In addition, the pronoun you does not change form from singular to plural.

Examples:

Jack, you may have some cake. (singular)

Children, you may have some cake. (plural)