1. The Bhikkhu Paṭimokkha
2. The Satipaṭṭhāna Sutta
3. The Mahāparinibbāna Sutta.
1. Because it was Subhadda's suggestion after the Buddha's demise that the bhikkhus could now do as they wished, and it was this that led to the convening of the First Buddhist Council to preserve the Dhamma while it was still intact. The Vinaya was recited first, as it is the life-blood of the Sāsana, and the bhikkhus are the guardians of the true Dhamma. If the bhikkhus neglect to observe the Vinaya rules, the Buddha's true teachings will soon disappear.
2. Using the right method is crucial to gain insight. The Satipaṭṭhāna Sutta includes all of the important teachings for the correct and diligent practice of mindfulness. There is no way to develop insight without first establishing mindfulness.
3. The Mahāparinibbāna Sutta is a chronology of the last six months of the Buddha's life. In it, he reiterates the essential points of his teachings again and again. For example, take the Dhamma as your dīpa
— your guiding light¹ and refuge — take no other refuge. That is, follow a wise and well-behaved teacher by all means, but don't let it become a clique or a cult. Think for yourself, study the texts, and observe the prescribed morality for Buddhists, whether lay or monastic.
¹ Often translated as "refuge," but I think a better translation in this context is "light." The word saraṇa = "refuge" is also used in the same passage:
attadīpo viharati attasaraṇo anaññasaraṇo, dhammadīpo dhammasaraṇo anaññasaraṇo.