Catullus 43

In this poem, Catullus describes a girl who many believe to be beautiful, even comparing her to Lesbia. It is a poem that delivers a bitter attack against what Catullus believes is a "tasteless and crude age." It is also a poem that defends his beloved Lesbia.

He saultes the girl and talks to her directly, showing that he probably knew her in person. Catullus employs the device of litotes heavily in this poem. He takes many of the girl's features, and denies their beauty by stating their opposite. She has a nose that is not small, a foot that is not beautiful, and eyes that are not black. By declaring her features to be the opposite of beauty, Catullus reinforces the fact that he finds the girl ugly.

Perhaps the poem is a criticism of the decoctoris amica Formiani, who is clearly disliked by Catullus. And because she is the friend of said man from Formiae, he also must be a jerk face who needs to be put in his rightful place by Catullus.
Finally, the poem could just be a defense of Lesbia. Catullus is so in love with her that he can't have anyone believing that there might be someone beautiful than her. That's some strong devotion.