It's = It is

Its = possessive, akin to his, hers, theirs.

It's the best.

It is the best.

Its trouble is a bad engine.

The '20s

He earned four A's and two B's.

Women's rights

The rights of women

The girls' toys

The horses' stable

Mathematics' rules

Measles' symptoms

The two deer's tracks

One corps' mission

The church's roof

The VIP's seat

States' rights

Descartes' theories

Fox's den

Justice's verdict

Marx's theories

For appearance' sake

For conscience' sake

Who is the winner

Whose towel?

The attorneys general

The attorneys general's lawsuit

Fred and Sylvia's apartment

Fred and Sylvia's stocks

Fred's and Sylvia's books

Teachers college

Teamsters union

Farmers market

Writers guide

Workers' compensation