I'm a big fan of the OotP film, and all of them in general. I love how cohesive they are, mostly stemming from the stability of the cast over the years. It's like they're all parts of onw big long story and it's been amazing to watch the kids grow, in many ways, as the films have gone on.
I even enjoy the earliest films, for what it's worth. I think all of the kids have been great finds for their roles, and just to ramble on for a moment, I particularly enjoy Rupert Grint's Ron in the first film. When they're getting their broom lesson and Ron's broom smacks him in the face and Harry laughs, I love the way he says "Shut up Harry", like they've already been great friends for a long time.
On another note it was just great seeing the kids be kids in the earlier films as well. Not the best of actors perhaps, but they all did a spectacular job as far as I'm concerned. One tidbit I'm particularly fond of is in the Chamber of Secrets film. While it takes me completely out of the film, I love the scene where Lockhart lets the pixies out of their cage and all hell breaks loose in the classroom. Obviously, the pixies aren't REALLY there, so you've got a room full of kids waiting for their cue to lose it. They get the cue and chaos ensues, but I particularly love to keep my eye on Emma Watson here who deliberately swishes her arm across her desk swiping everything to the floor. It's ridiculously awesome! And I have to look at it again, but I believe she looks as though she's trying not to crack up as well.