Hmmm. I liked 6 because there was a good mystery there of the whole plot and Draco and the "twist" and and and.
I liked 7 because of the whole, FINALLY, it's wrapped up. You know what bumps that down, though? The 19 years later or whatever it was at the end. It read like freaking fan fiction. Hated it.
Of the movies, though, 3 is probably my favorite. Nicely paced, interesting time theme throughout, and you know, Gary Oldman.
I'm glad the series was written, and it is one that I can revisit over and over again and wish that I was a magical boarding school. And, you know, British. :)