One of my favourite lines was in Hamlet when Queen Gertrude says "the lady doth protest too much methinks."

From wikipedia:

The line, like most of Shakespeare's works, is in iambic pentameter. It is found in Act III, Scene II of Hamlet, where it is spoken by Queen Gertrude, Hamlet's mother. Hamlet believes that his father, the king, was murdered by his uncle Claudius (who then married Gertrude). Hamlet decides to stage a play, the Murder of Gonzago, that follows a similar sequence of events, in order to test whether viewing it will trigger a guilty conscience on the part of Claudius.

As Hamlet, Gertrude, Claudius and others watch the play-within-the-play, the Player Queen, representing Gertrude, declares in flowery language that she will never remarry if her husband dies.[1] Hamlet then turns to his mother and asks her, "Madam, how like you this play?", to which she replies ironically[2] "The lady doth protest too much, methinks", meaning that the Player Queen's protestations of love and fidelity are too excessive to be believed.[1][3]