In Tennyson's poem The Eagle, certain things about the eagle are stressed based upon the form and rhyme of the poem.
For example, in the first line, Tennyson is stressing the eagle's hands: "He clasps the crag with crooked hands." Here, to things about the eagle are being stressed: his strength and his human-like characteristics (personification: giving "hands" to an eagle).
His strength is again identified in line two when it is stated that he can survive in in lonely lands. It is the eagle's ability to survive in solitude that is stressed.
Again, in the fourth line, the strength of the eagle is stated.The sea is "crawling" beneath him. Both the word crawling and beneath speak to the eagle's superior nature.
Therefore, overall, the one quality of the eagle which outweighs all others is his strength.