I personally like the 3rd one the best. I'm the biggest Harry Potter fan ever! Lol. I'm obsessed with it as much as I am ALIAS but I have more Harry Potter stuff cause there isn't as much ALIAS stuff out there. I have read all of the HP books 5 times each. 3rd is the best because in the end he doesn't get to Voldemort like in all the rest. It is different. And because of the plot twist too! It is awesome! This is weird cause I'm actually listening to the Prisoner of Azkaban soundtrack rihgt now! Lol.
I have to say 5. A lot of people didn't like the ending, but you kind of have to read into it. I used to like the third the best, but I read the fifth several more times, and came to like it better. All these sublties you don't notice the first time pop out at you, and it makes it a much more enjoyable book.
Personally, I've never liked Sirius as a character -- too whiny, unstable and rather immature (although I understand why he's that way). As much as I can appreciate what he represented to Harry, I don't think he (S) would have been a great influence on him (H).
But beginning with PoA and then escalating through the last 2 books, the HP universe really took off for me. I *lurve* the Harry!Angst, and the difficult choices that many of the characters have to make. For me, the delicious meat of the story is all Harry, Dumbledore, Snape and Voldemort. The rest are just the flavorful side dishes -- they help to complete the meal, but they don't fill you up