You ask a question that is difficult answer, since it is based on perspective. My opinion may be different from that of someone else.
In my opinion, the greatest thing that Ponyboy says is "He ain't a Soc...he's just a guy." This one quote reveals just how much Ponyboy is maturing in the way he views the world. At the time that he says this, he is beginning to truly realize that, though the Socs and the Greasers are enemies, they are not very different, after all. It is a profound message.
To me the best thing Ponyboy said is [not quote to quote but paraphrase]. That you can see the sunset on both east and west side.
I think the best thing Ponyboy had said in my perspective, is the part when he tells Cherry Valance that you can see the sunset real good from the West side but also real good from the East side too. This part in my opinion was the best thing he ever said