Diligence in Study
Shabbos & Yom TovTorah learning
Rav Yechezkel wrote, "The main thing is diligence in study." He taught that one must learn Torah works of true meaning: Mishnayos with Tosafos Yom Tov, the Gemara, the Poskim, Chumash, Nevi'im and Kesuvim, and also works of mussar.
Source: Writings of the Noda B'Yehuda