I am torn between 4 and 5. I picked 4 because you got to learn a lot more about the world the story takes place in. I like 5 because of the amount of development there is for the background characters.
Going for 5 and 7. I loved all the Order of the Phoenix stuff going on with all of them. I love Sirius and I loved Remus. I thought I wouldn't like Deathly Hallows because they weren't gonna be at Hogwarts doing magic and shit, but I actually ended up loving it a lot.
I loved the last book. Though I cried through just about most of the book, it was still one of the most emotional reads I've ever had. I loved these books and the whole series was fantastically written.