To some extent, the answer to this question depends on your personal opinion. However, you must keep in mind the purpose of this scene as you try to think about which line is the most important. This scene is really about establishing the personalities of the craftsmen who will be performing in the play within a play. Since the most important of these characters is Bottom, the most important line should be one that reveals something about his character.
Bottom has many lines that show what a buffoon he is. You could pick almost any one of them. My pick is this line:
I will aggravate my voice so, that I
will roar you as gently as any sucking dove; I will roar(75)
you an't were any nightingale.
This line shows perfectly who Bottom is because it is a silly concept to begin with (he'll roar gently while playing a lion so he doesn't scare the women) and because he makes mistakes ("sucking dove") while doing so. This really shows us what his character is like.