Antithesis

Antithesis: The term used to refer to an author's use of two contrasting or opposite terms in a sentence for effect. The two terms are set near each other to enhance or highlight the contrast in opposite meaning.

Antitheses are used to strengthen an argument by using either exact opposites or simply contrasting ideas, but can also include both. They typically make a sentence more memorable for the reader or listener through balance and emphasis of the words.

When he became the first man to walk on the moon, Neil Armstrong said that it was a "That's one small step for a man, one giant leap for mankind."

"It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness." Charles Dickens, A Tale of Two Cities

My Example:
"It was the best experience, but the worst experience."
"He was the best player in the team but also the worst player in the team."