I can't actually choose, there all GREAT but, Harry Potter And The Philosopher's Stone, cause the adventure starts there, and then it has this line : Yer a wizard, Harry!!
My favorite has to be The Deathly Hallows, not only because it brings everything to an end but because is the perfect sum of action, romance, friendship, suspense and sadness <3
My favorite is Prisoner of Azkaban because, when re-reading it, it’s fun to see Harry’s reaction to Sirius’s animagus following him around and we know something important that the characters don’t, which is fun.