Study in Depth
Rav Menachem Mendel often warned his chassidim to study in depth (iyun). He praised this form of study, when done lishma (sincerely, for the right reasons), as one of the forty-eight ways through which the Torah is acquired, and he taught that all the tzaddikim of his generation reached lofty spiritual levels only through study b'iyun.
Source: On Avos, chapter 6 (the forty-eight ways of acquiring Torah)