My personal favorites

Confessions of a Crap Artist

A critical look at married life that manages to be both funny and disturbing. The use of varied points-of-view (a PKD trademark) really makes this novel work. Out of PKD's mainstream novels, this is probably my favorite.

Do Androids Dream of Electric Sheep?

The inspiration for Blade Runner. If you like that movie, you should read this book. If you don't like that movie, or you've never seen it, you should read this book anyway, because the book's much better. A true classic when it comes to cyberpunk and android fiction, or science fiction as a whole. Through its film adaptation, it's arguably the most influential science fiction book ever written.

The Man in the High Castle

This Hugo Award-winning alternate history novel depicts normal American life... after America lost WWII and is divided between Japan and Nazi Germany. Very subtle and accomplished writing.

Martian Time-Slip

A retro Mars-based novel (retro because we're on the surface, we can breathe, and there are indigenous Martians) about schizophrenics, education, and the perception of time. This has a nice mix of science fiction and horror. Highly recommended if you like The Sixth Sense or Stephen King's writing.

A Scanner Darkly

Dark, gritty, and incredibly funny, this autobiographically-inspired novel is a very powerful portrayal of the 70's drug culture and the symbiotic relationship between the drug industry and the rehab industry.

The Three Stigmata of Palmer Eldritch

An LSD trip in book format, this novel deals with drug use and the creation of fake realities. The only problem: if your fake realities are convincing enough, how do you know when you've returned to true reality? If you like The Matrix, Dark City, or eXistenZ (which actually contains a reference to this novel), you'll like The Three Stigmata of Palmer Eldritch. (This is the first PKD book I read.)

Ubik

A humorous science fiction detective story about higher (and lower) levels of reality and time regression, with some Platonic philosophy thrown in for good measure. It has twists that rival those in the movie The Sixth Sense.

Valis

Part autobiography, part philosophical treatise, this is a very dense and funny book detailing Philip K. Dick's direct encounter with God in 1974. Or was it aliens? Or the Russians? Or the CIA? This is a must-read for Philip K. Dick fans; newbies should read at least one or two other PKD books first.