I would have to say OOTP.
In fact i'm re-playing the game at the moment because i finished HBP within a matter of a few hours.
The castle and grounds have been expanded for HBP so there is more space to roam around and the graphics are indeed better.
The potions and Dueling and Quidditch are fun to begin with but even they can get repetative after a while and i much prefer the map and footprints method for OOTP than having Nick as the guide.
I know there was no Quidditch in OOTP but there was a bit of Dueling and there was so much more to do with finding all the DA members and performing tasks to get them to the Room of Requirement, the mini tasks for all the teachers and then completing all the extras to get 100% for each room or part of the castle.
I really enjoyed playing the different Gobstones games too and i wish they had included these in HBP but alas not.
So it's definitely OOTP for me
