I think that Ponyboy would suggest that his own sense of maturation and growth brings him the most amount of pride. Ponyboy clearly recognizes that his experiences have helped forge his identity. He understands that he has remained true to his own notion of self without it being overtaken by the cruelty and callousness of the world around him. Ponyboy has not surrendered the character within him. He has found a way to emerge from the socially stratified world where people are reduced to caricatures to become a thinking and unique person. Johnny tells Ponyboy that he needs to "stay gold." Indeed, this characteristic is where his pride lies.
At the end of the novel, Ponyboy stands up for the Greasers. It becomes clear that he will never surrender this part of his identity. When he breaks the bottle in a threatening manner, it reflects how he might succumb to the violence and division of the world around him, suggesting that "nothing gold can stay." Yet, when he cleans up the shards so that no one will get hurt, it becomes clear that he has remained "gold." He is pure. Ponyboy is still the person who watches "the same sunset" and is struck with awe by its beauty. He holds pride about the fact that he is a part of the world, but remains distinct from it. He is his own man, and yet finds a way to "stay gold." As Ponyboy acknowledges the need for social solidarity with "I reckon we all just wanted to stay together," it is also evident that he emerges as a complete individual with his own identity and sense of self within a complex and intricate world. It is this affirmation of the world and his place in it towards which Ponyboy feels pride.