It is a very good question, because there is presently a wealth of papal preaching and teaching available to us; it is also a difficult question, because there is presently a wealth of papal preaching and teaching available to us.
That being said, after browsing around a bit, I have selected these, in no particular order:
- Deus caritas est (on Christian love), by Pope Benedict XVI;
- Haurietis Aquas (on devotion to the Sacred Heart), by Pope Pius XII;
- Fides et Ratio (on the relationship between faith and reason), by Pope John Paul II;
- Humanae Vitae (on the regulation of birth), by Pope Pius VI;
- Nono Millennio Ineuente (at the close of the great Jubilee of the Year 2000), by Pope John Paul II;
- Salvifici Doloris (on human suffering), by Pope John Paul II;
- Evangelii Nuntiandi (on evangelization), by Pope Paul VI;
- Mystici Corporis (on the Mystical Body of Christ), by Pope Paul VI;
- Address to the Roman Curia, by Pope Benedict XVI; and,
- Redemptor Hominis (on the Redeemer of man), by Pope John Paul II.
To be sure, I have not read every available papal document. If I missed one that you would have placed among the top ten, please suggest it by leaving a comment.