The Outsiders. What are the most important one or two lines of chapter 12 and why?

This really depends on what you feel is important to the themes and characters in the novel. I think that Ponyboy finally comes to accept that Darry wants the best for him and is trying his best to keep the family together,

Darry and I would probably still have misunderstandings – we were too different not to – but no more fights. We couldn't do anything to hurt Soda.

Johnny leaves a deathbed letter asking Pony to help Dally be less angry and disillusioned,

There's still a lot of good in the world. Tell Dally. I don't think he knows. Your buddy, Johnny.