This topic is just impossible. I have purchased hundreds and hundreds of books from bookshops, nearly all of which were great finds or I wouldn't have bought them.
I guess if we're going for a "wow" find, something rare or surprising, I will go with Paint It Black. Not that the book was rare or hard to find, but it is rare for me to walk into a bookshop and plonk down the full hardback price of a new release. I loved White Oleander so much that when I ran into Janet Fitch's next novel (not knowing there even was such a thing in existence in my pre-Goodreads days), I grabbed it up without a second thought. And it was totally worth it.