My number one favourite moment from DH has to be "Think of something happy... we're still fighting" because this time Luna's understanding actually saved Harry's life. He was broken, he lost so many people he loved, he wanted it all to end, he had absolutely no hope left - and then there is Luna's voice and Luna's words and the hope magically returned again, no matter how dark it was around him, she brought back the light into his life and made him want to fight again.