I'm not sure there is only one, unique answer that is universal to all people and circumstances.
Depending on the person, and the time of their life, they might benefit from different practices. That's partially why we offer so many different practices in this tradition--so that people are able to choose what they most need in the present moment.
Moreover, the practices are mutually supportive. So your work with one practice will help you with the others, and your work with the others will help you with the one.
For some people in a given moment, reading the books may be what they need. And Samael Aun Weor
recommends this. Others may benefit from silent meditation, but find that at other times what they need most is mantras or prostrations.
It is helpful to get experience with a lot of practices so we can see how they work, and just as important to learn how to look inside so we can see what we need.
So I don't think there is one "practice" per se, that is the be all and end all. However, there is an attitude (or better said, an understanding) that should underlie our practices. Your question reminded me of a story I heard once, which would be useful to recount here, since I think it gets at the root of what you are asking...
Seeing a monk circumambulating a monastery one day, Dromtonpa [Tibetan Buddhist master, and student of Atisha] called to him, “It is good to circumambulate, but it would be better to practice Dharma.” The monk thought, “Maybe it’s better to do prostrations.”
When he saw the monk prostrating, Dromtonpa said to him, “It is good that you are doing prostrations, but it would be better to practice Dharma.” When the monk tried reciting prayers and meditating, Dromtonpa again said the same thing.
Finally the monk asked Dromtonpa, “That what should I do?” Dromtonpa answered, “Give up on this life! Give up on this life! Give up on this life!”
You can read more about this story in the link below.