I believe the single biggest plot twist in film history, though it is now common knowledge and a part of pop culture, happened toward the end of The Empire Strikes Back.

Other films that had plot twists that I thought were well constructed:

The Usual Suspects
Se7en
LA Confidential

And of course:

Monty Python And The Holy Grail: the entire film had a plot that can only be described as twisted.

Take care all.