7th and 8th kept pretty close to the books to be honest, it only deviated on a few parts, Goblet of Fire and Half Blood prince were definitely weak entries to the series, i'd say Order of the Phoenix was so out of order it made me want to puke.I finally watched the last three Harry Potter movies and now I can say with certainty that the first three (along with the 8th actually) are my favorites. I don't remember a thing from the books so I can't really say how well the last books were adapted or not but I still enjoyed them. The Half-Blood Prince was pretty weak, though.
If I had to pick, I'd say my favorites are 2, 5, and 8.
Incidentally the person who plays umbridge hated playing her. she said that umbridge was truly evil