There are obviously a number of quotes that you could select, however, for me, I would want to pick a quote from the end of the scene when Hermia wakes up alone having suffered a rather disturbing nightmare. Consider what she says and how it relates to her situation:
Methought a serpent ate my heart away,
And you sat smiling at his cruel prey.
What is ironic about this dream is that it actually reflects reality. The dream expresses symbolically what has actually happened in real life, for the magic flower has caused Lysander's affections, which he claimed to be so secure and unmoving, to completely change his attitude towards Hermia so that he is completely indifferent to her, or even hates her. Thus the image of Lysander smiling as a serpent devours Hermia's heart accurately parallels the change in Lysander's affections that has just occurred. Hermia's heart will be devoured as a result of Lysander's change.