Choosing Elizabeth Proctor's most important quote is left up to individual readers; therefore, the answer to this question is subjective. I like the following lines from act 2.
It is her dearest hope, I know it. There be a thousand names; why does she call mine? There be a certain danger in calling such a name—I am no Goody Good that sleeps in ditches, nor Osburn, drunk and half-witted. She’d dare not call out such a farmer’s wife but there be monstrous profit in it. She thinks to take my place, John.
This is an important quote from Elizabeth to John, because she flat out gives John a legitimate reason for why Abigail is doing what she is doing and why she would give Elizabeth's name. John did commit adultery with Abigail, and John believes that he has adequately ended all possible relations with Abigail; however, Elizabeth gently explains that Abigail sees his physical intimacy as a promise to her. Elizabeth correctly points out that Abigail is motivated by jealously and selfish gains.
I also think the following lines are also important from Elizabeth.
John, it come to naught that I should forgive you, if you'll not forgive yourself. It is not my soul, John, it is yours. Only be sure of this, for I know it now: Whatever you will do, it is a good man does it. I have read my heart this three month, John. I have sins of my own to count. It needs a cold wife to prompt lechery.
Do what you will. But let none be your judge. There be no higher judge under Heaven than Proctor is! Forgive me, forgive me, John—I never knew such goodness in the world!
John has worked very hard to earn back Elizabeth's trust in him. He desperately seeks her forgiveness, but she has been averse to fully giving it. Here we see that she openly forgives John, and we see her confessing her own sins to her husband. Her forgiveness and honesty causes John to seek his life and confess. He wants to be with Elizabeth and his family even if it means living with his lie.