My favorite right now (it always changes each time I read the series!) is Prisoner of Azkaban. . . POA is the last "light" book before the death and doom of the other books. There's no Voldemort (with the exception of the threat of him, as always), there's his first trip to Hogsmeade, getting the Firebolt, and Harry learns who Sirius is. For that brief moment where he thought he could go and live with him, it was his happiest time! I fear Harry hasn't had many other of those moments, nor will he in the imediate future.