I would say The Order of the Phoenix. Also my favorite installment of the series. He's really pushed to the limit. Dealing with Cedric's death, his alienation from the people he thought were his friends, the connection with Voldemort that makes him question everything he is, and finally he's forced to face his doubts when he's given the chance to kill Bellatrix after she kills Sirius. Is he the hero that everyone thinks he is? Is he really any different from Voldemort? His character was brilliantly depicted in that book.
I think we learn the most about his character in the 1st book actually. We see who he chooses for friends; and who he doesn't. We see how he reacts to teachers, and learn he takes after his dad in Quitich(can't spell it). We learn for the first time how brave he is(to face his parents killer at 11 years old!). We learn ALOT of things about him in the 1st book.
best answer
