I hear you, I love three just because its when I really started to get hooked onto HP (and then, it was something that bonded me and my mom which was nice). But if I had to pick one without sentimental value probably Goblet of Fire, it's when garbage hits the fan for poor ol' Harry.
I would have to say Harry Potter and the Sorcerer's Stone. I love the beginning because it gets you tied into whats about to happen and all the gossip so to speak. I think without a good beginning, it would have flopped.. but we've all seen how successful it has become.