Which figurative language is being used in the line Thou art more lovely and more temperate?

Metaphor.

Explanation:

Metaphor is a figurative language used to compare two distinct things or ideas not with direct meaning but in an implied manner.

The given line is taken from the Sonnet 18 of William Shakespeare.

The speaker is comparing his lover to the summer's day in an implied manner. The speaker is trying to assert that his lover is more lovely and temperate than the summer's day. In this line, the speaker is admiring the beauty of his beloved. He is saying that though a summer's day is lovely but his lover is more lovely and temperate that it.

So, metaphorical figurative language is used in the given line.