The Pāli canon contains many thousands of discourses (they are even difficult to count exactly, 8000? 10000? more?), spanning from a few sentences, to fairly long pieces of many pages, but mostly in the range of some thousands of words.
The discourses were not meant to be read as a book or as an entire collection. They were pronounced in times where culture was an entirely oral affair, based on listening and remembering. It is thus likely that many discourses are intended to be more or less self-contained. Knowledge of the whole corpus was not required. Each discourse is are like a seed, with the potential of giving rise to the whole teaching, if duly stimulated (investigated).
To get started, there are at least five discourses that are very representative of the Buddha's teachings and of his key interests.
(1) The discourse on the simile of the arrow (SN 36.6) draws attention to a fundamental insight concerning the three bases of intentionality that, in the Buddha's view, should be abandoned, namely, greed, aversion, and ignorance. In this discourses, the Buddha shows why aversion is in a sense more fundamental, and how the other two emerge as a way of dealing with it.
(2) The discourse on friendliness (Sn 1.8) illustrates a way of practice aimed at abandoning aversion, and how doing so leads to greater ease and clarity, which might even result in ultimate freedom. This is also a reminder about how deeply entrenched are one's own training and a more compassionate attitude towards all living beings.
(3) The discourse on the five themes for frequent recollection (AN 5.57) provides a simple and yet extremely powerful synthesis of one way of approaching meditation, which starts from a verbalized contemplation of a certain theme, and then develops its practical, moral, cognitive and emotional implications. It shows how reflecting on apparently depressing topics (aging, illness, death, separation) can be a source of happiness and release.
(4) There are several discourses in which the Buddha explains what he realized during his own awakening. A particularly powerful one, is 'The World' (Ud 3.10). Here the Buddha introduces several of his main topics of reflection and teaching: the need to abandon greed, aversion, and ignorance; the pointlessness of self-concern, the necessity of dismissing the the duality between existence and nonexistence, and the process of conditioned co-origination.
(5) The Buddha is epitomized as the ideal teacher, which means that he knew (or learned) how to present his insights to the audience in such a way to establish a connection with them and eventually lead them to grow. This skill is apparent in what is perhaps the most important of all the discourses, the 'Setting in motion of the wheel of Reality' (SN 56.11), which encompasses the whole teaching and its development, by also outlining the DNA of the Buddha's training.
This discourse is more subtle than what might seem at a first superficial reading, and it is also composed with a polyphonic and symphonic style that makes it particularly beautiful.
In traditional Buddhist communities, this is one of the discourses that is also fairly regularly chanted.