Final answer:
The skylark's greatest quality, as described by the speaker in 'To a Skylark,' is its joyful and liberating song that symbolizes an ideal of perfection.
Step-by-step explanation:
In the poem "To a Skylark," the speaker extols the skylark's greatest quality as its unabashed and joyful song. The skylark's music represents an ideal of perfection and pure joy that seems untainted by sorrow or any form of human suffering. Through its song, the bird is able to reach a level of spiritual transcendence and beauty that humans can only aspire to, being both a symbol of freedom and an embodiment of nature's simple, unencumbered happiness.