Who are the most important characters in Beowulf?

The most important characters in Beowulf are the titular hero, King Hrothgar, Wiglaf, and the three monsters Beowulf faces, Grendel, Grendel's Mother, and the Dragon.
Beowulf is significant because he is the hero. He represents true heroism and the epitome of the warrior-king once he takes a crown.
Hrothgar serves as a role model for Beowulf early in the story. He prefigures the wise warrior-king Beowulf shall one day grow to become. Hrothgar is generous to his warriors and concerned for his people. He gives Beowulf a fatherly speech warning him against being too prideful and reminding the young warrior of his own mortality.
If Hrothgar is a father figure to Beowulf, then the warrior Wiglaf could be seen as a son figure or at least as the closest thing Beowulf gets to a successor. When the other warriors prove unwilling to join Beowulf in the final battle against the Dragon, Wiglaf alone stands by his king's side, both in the battlefield and as he dies from his wounds.
The monsters Beowulf fights serve as the chief antagonists. Each one is harder to conquer than the last and Beowulf must find different ways of besting them individually. Grendel is a predator made bitter by his outsider status (he is descended from the biblical Cain, who was doomed to wander the world separated from the rest of his family), so he takes his anger out on human communities. Grendel's Mother seeks vengeance on Beowulf after he kills Grendel in hand to hand combat. Unable to take her on with his fists alone, Beowulf uses an enchanted sword to defeat her. The Dragon attacks Beowulf's people after he has grown old. He fights the Dragon and wins, but at the cost of his own life. While all different, the three monsters are united in their antisocial nature and serve as threats to organized society.
While there are other characters in the epic poem, these are the most important in terms of thematic significance and the closeness of relationships with the protagonist, Beowulf.