For satisfying, I'd have to say the first book.  Whenever you read the series straight through, it gets darker and darker so gradually that you almost don't notice it happening - and then you go back to book 1 and it's like "WHOA HEY, SUNSHINE AND LIGHT!"  So yeah, Harry Potter and the Sorcerer's Stone has all that newfound wonder and charm and literal magic of the wizarding world, so I have to say it's the best as far as a standalone read goes. 

But I personally most enjoy The Order of the Phoenix, for various reasons, including but not limited to my top 5:
1. It's the longest - so when I was in college and agonizing over which representative of the collection to bring, I decided that had the most bang for my buck even if I paid for it in weight

2. It's full of Sirius and I adore anything that involves him and/or his creepy old cursed house (although Christmas is the best) - and as far as deaths go in the series, I'd say he's got the most powerful

3. It's got the delightfully creepy Umbridge, whose scenes are some of the most fun to read even as they creep you out

4. Snape + occlumency lessons = win!

5. And best of all, TONKS.  Being unequivocally awesome at every turn and cementing her first-place position in my heart.  Yeah, I'm pretty sure she's at least 50% of why this book is my favorite.
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In other news, I got nothin'.  Except that second ER fic I've had in storage for over two years that looks like it might actually get done by the end of summer, so I'm off to go work on that.  (It's exciting stuff, me + fanfic.  My new goal is to publish one whole story a year.)  And may I just say, as far aiding me with said fic, chloegreenfield.com continues to be an excellent YouTube presence, handily compiling all of Sarah's scenes on an individual channel.