I think HBP is the weakest. Here's what I would've altered for the films, screenplay-wise.
HBP: Take away the attack on the burrow and replace with a good Harry/Ginny kiss, Voldemort backstory, and Harry/Snape loathing. Even if it makes it a worse film, it'd be a better setup to Deathly Hallows.
Personally, I loved the "mood" they gave HBP. HBP is easily my least favorite of the books, but somehow they made me really enjoy the film. Broadbent was especially good as Slughorn, and made me like that character more than I did before.
The only thing I regret about the sixth film is, like you said, the lack of Voldemort backstory. It would've helped if they had had at least one more memory, but I think they're saving all the Horcrux info. for the last film, so audiences will know what's going on right in the moment. I also feel the lack of the Dursleys in both HBP and DH is a huge shame. That said, I thought the cut of the final battle at Hogwarts in HBP was a good thing. Since we'll get another battle at the school in the final film(s), it would be a bit repetitious. The only real addition I didn't like was the very beginning with Harry flirting with some random girl.