I have no idea which of Philip K's works is the "best" – I'm sure they all have their strong points and weak points, since he was such a brilliantly dopey and consistently inconsistent author. I think
Scanner darkly may be one of his strongest efforts on the whole, although it's locally marred by the superfluous and weak ending. On the other hand,
Ubik is an absolute mess as a narrative-driven novel, but it contains stark and powerful nightmare sequences that would have made a great short story in their own right. As a general rule, you can pick up any of his works and find quite a few striking passages, even if the book in its entirety is mostly crap.
I've also enjoyed his later non-SF works for the unbridled kookiness and cracked erudition:
VALIS reads like a scholarly essay on the ideas of Gene Ray and Dr. Bronner, written by someone who has swallowed entire library aisles of ancient philosophy and Gnostic apocrypha along with reams of blotting paper. It's also semi-autobiographical, which makes it a rather eerie reading experience at times.
Transmigration of Timothy Archer is also good: it draws on the same reinterpreted Spiritistic-New-Age-Acid-Christian mythology as the VALIS trilogy, but unlike most of his later writings, it's also a well-rounded work of fiction.
Pretty sure Philip K. Dick sang Blade Runner, or maybe I'm mixing up my poetry.
it was david bowie, you blasphemous philistine
EDIT:
I read most of Philip K's novels in my early teens, and
The Man in the High Castle was one of those that I filed under "forgettable."