I don't think there is a best. They all did a fairly good job. I'm partial to OoTP and HBP, but I wouldn't immediately think of them as the best adaptations. They're close and are good narratively, but they still miss out some pretty good pieces of information.
If I had to pick, I'd say Philosopher's Stone. The book itself is pretty whimsical and not much happens up until the second half, when Harry eventually gets to Hogwarts. It did a good job of missing the first chapter - we establish with location the Dursleys appear to lead very normal lives and keeping it from predominately Harry's perspective is really effective. The sets, the score and the characters gestures say a lot whilst also saying very little. As the start of the series, I think it's very fitting and a great set up for the other films.
So whilst I don't necessarily think there's a best adaptation, or want to sit through a 3+ hour film for the sake of every chapter or minor detail happening, I think Philosopher's Stone is one of the better ones.