I’ve read four from Pope Francis, and personally recommend all of them:
The Joy of the Gospel (Evangeli Gaudium), his first apostolic exhortation on evangelization,
On Care for Our Common Home (Laudato Si), his encyclical letter on environmental stewardship,
Rejoice and Be Glad (Gaudete et Exsultate), another apostolic exhortation on our call to holiness, and,
The Name of God is Mercy, a conversation in book form with Andrea Tornielli.
I can’t say which is my favorite. There are very good teachings in all of them.