The Three Marks of Existence are anicca, dukkha, and anattā, or impermanence, unsatisfactoriness, and not-self. Impermanence is the principle that life is always changing and cannot be controlled by man; a fundamental, underpinning principle of the Buddhist faith.
Impermanence is arguably the most important mark of existence as it is applicable to everything; to the whole movement of the universe and of human life. Impermanence permeates all aspects of life, even in inanimate objects, and thus is a constant reminder of the powerlessness of man. An understanding of impermanence motivates Buddhists to improve their quality of life; to achieve enlightenment.
One could argue that Dukkha is a more important mark of existence. This is the idea that there is always a general discomfort or dissatisfaction in life, this is important to Buddhism as the fundamental principle of Buddhism is to escape suffering through enlightenment, such as annica represents. Dukkha however is more explicit in the suffering of man.
Annata may also be the most important in that it addresses the issue of identity of the person and the illusion of self as the main barrier to enlightenment.
We can conclude however that all three marks are essential as they portray the whole meaning of life. Without any of these, the underlying principles that Buddhism addresses are incomplete.