Roald Dahl is the unsurpassed master of plot twists. If you've read stories like "Skin", "Lamb To The Slaughter", "The Way Up To Heaven", "Mrs. Bixby And The Colonel's Coat", or even the very common "The Umbrella Man", you know what I'm talking about. Of course, My Uncle Oswald is simply fabulous as well - coarse yet so humourous the vulgarity is forgivable! (Simply a must-read, this one.)
Most recently, I loved the plot twist in Jeffrey Archer's Only Time Will Tell.
I couldn't decide on a particular story or book to put in here, there are just so many contenders, thanks to Mr. Dahl! So sue me for cheating. :|