I'd have to say the fourth and fifth for the book and the fourth and the second part of the seventh for the movie. Don't ask me why I like those because I don't really know for sure. I simply enjoyed reading/watching those books/movies more than all the others.
I only read the first few and then got lazy and started watching the films For a children's book series I have to admit Harry Potter sucks you in. I loved that kind of stuff as a kid so this was an experience similar to reliving my childhood in many ways.
I loved the 1st, 3rd, 4th, 6th and 7th books and I never watched the movies.