Dukkha [all unhappiness], its origin, its cessation, and the way to its cessation -- these are the Four Noble Truths, the "elephant's footprint" that contains within itself all the essential teachings of the Buddha.
It might be risky to say that any one truth is more important than the others since they all hang together as an integral unit. But if we were to single out one truth as the key to the whole Dharma [Buddha's Teaching as a whole], it would be the fourth Noble Truth, the truth of the way, the path to the end of dukkha. And what is the path? It is the Noble Eightfold Path, the way made up of the following eight factors, which are divided into three larger groups:
We say that the path is the most important element in the Buddha's teaching because the path is what makes the Dharma available to us as a living experience. Without the path the Dharma would just be a shell, a collection of doctrines without inner life or the ability to propel us to happiness. Without the path full deliverance from suffering, or unhappiness, would be a mere dream.
*Note on the word "right" (samma) in front of each term: It indicates that not any, for example, concentration will do. All of the expressions used here are technical terms with careful definitions to be unfolded in the discourses (sutras) and the commentaries (tika). One could spend a lifetime pursuing what s/he thinks constitutes "concentration" and get nowhere in terms of the Buddha's path. The Buddha defined "right concentration" as proficiency in the first four jhanas (absorptions). The Path of Purification (Visuddhimagga), a compendious commentarial work, goes into greater detail. "Right" means correct, effective, balanced, specific, defined, harmonious, as taught by the Awakened One. A shallow reading of the Dharma that leads one to grasp it wrongly can be quite a dangerous thing. Understanding that all Buddhist terms unfold and are never left unexplained or uncommented on saves us from resorting to our own biases, preferences, and opinions.