Why are Beowulf and Hrothgar both considered great kings?

For the sake of reference, I will be using Seamus Heaney’s translation of Beowulf to answer your question.
To understand what standard the Anglo-Saxons considered to be “great kingship,” the anonymous author of Beowulf establishes this for his readers in the first lines of the poem by describing Shield Sheafson. Shield accomplishes a number of things, but is known primarily as a “wrecker of mead-benches” to whom “each clan on the outlying coasts beyond the whale-road had to yield… and… pay tribute.” The passage ends with “That was one good king.” This information all appears in the first eleven lines of the poem.
So that being said, a great king is one that expands his borders, inspires terror in his enemies, and is a brilliant warrior who destroys the halls of other kings. The Anglo-Saxons made their names in their endless blood-feuds between kings, and the prize was always the treasure of others, namely gold. Heaney writes in his introduction that

Gold is a constant element, gleamingly solidly in underground vaults, on the breasts of queens or the arms and regalia of warrior on the mead-benches. It is loaded into boats as spoil, handed out in bent bars as hall gifts. (xvii)

A great king gains gold by defeating other kings, and can only do this by becoming a great warrior in his own right. Then, after achieving the gold spoils of war, he distributes it to his thanes according to their contributions to the battle. Therefore, one can easily identify a great king in Anglo-Saxon culture, a highly merit-based society, by the amount of gold in their possession, as well as their thanes’. Hrothgar’s thanes possess gold rings and jewelry, and Heorot is described as one of the most impressive halls of its day. This gilded mead-hall is evidence of Hrothgar’s achievements as a king, but its subsequent destruction also becomes a symbol of his failure in leadership.
Wiglaf describes Beowulf’s tenure as a king before he goes to fight the dragon, showing the reader that he has been diligent in both attaining gold and distributing it to his thanes throughout his reign. The only creature in Anglo-Saxon culture that was meant to hoard gold is the dragon itself. Beowulf’s attempt to kill the great serpent is doubly an effort to protect his people and to gain its spoils. By the time Beowulf dies, he possesses the same notoriety as the legendary Shield.