Having read virtually every scifi book that PKD wrote, and some straight fiction by him as well, I'd have to say that for my taste he was at his best when he was writing straight scifi, and the more scifi there is in what he wrote the better it's likely to be.

While I'd agree that Ubik is a great introduction to the PKD universe, I would also classify it as his best book. I also love many of his other books, but to me none of them are as strong or consistently good as Ubik. Some of them have great parts, though, so I definitely wouldn't stop at Ubik, but I've yet to read a better book by him, or even one that was just as good.

I want to add, while my first paragraph may have made it sound that I didn't particularly care about the book, I am very glad that I read it. It was a weird experience, but one of the most interesting ones reading a book. It made me continue to think about it, obviously.

VALIS is probably the most interesting book on religious experience I've ever read, fiction or non-fiction.