Shulchan Aruch HaRav and Rebbe Rayatz of Chabad on what to study.

"Every Jewish male — whether poor or rich, whether of perfect health or ailing, even a poor man who begs from door to door, and even one who is married and has children — is obligated to establish a fixed time to study the Torah by day and by night, as it is written, 'And you shall meditate on it day and night.'

"During this time one must study the Written Law, and also the Oral Law, [which comprises both] conclusive legal rulings and Talmud. One should not restrict himself to Talmud if his Torah study is in its early stages, for initially one should know many of [the laws defining] what is forbidden and what is permitted, without [studying] their motivating principles and supports. [Moreover, one should learn] how to practice the mitzvos that are currently incumbent upon us, so that he will know how to observe and perform and will not sin, [even] before he knows everything properly from an in-depth study of the Talmud. (Shulchan Aruch HaRav, SECTION 155 The Universal Obligation to Study the Torah. (1-2))"  Chabad.org


Sometimes it seems as if some people deem Chassidus the only topic worthy of study but you see from the Baal HaTanya a very different picture of primary subject matter.

And now the Rebbe Rayatz:

"Everyone should study mishnayos by heart, according to his ability, repeating them while walking in the street. Through this, we will merit to greet Mashiach.

Chassidim must study Chassidus. Chassidim in general should do so on Monday, Thursday, and Shabbos. Temimim should do so for an hour every day."

HaYom Yom Tackling Life's Tasks - 21 Kislev

And then there's the Besht: "The Baal Shem Tov conducted a regular study session in Gemara with his disciples, combining acumen and erudition. It embraced the writings of Rambam, R. Yitzchak Alfasi, and Rabbeinu Asher, and other commentaries of the Rishonim1 that were relevant to the passage under discussion." (Hayom Yom: Tackling Life's Tasks - 13 Cheshvan)