Let's start the countdown.

In 10) ORPHAN

The Orphan is in fact a 40 year old murderous dwarf with a thing for married men. She is also the poster girl for Oil of Olay.

9) THE BOOK OF ELI

Eli has been blind all along! And "the book" he has been lugging around is The Bible - in Braille, which is why it's quite hefty. Didn't 'see' that one coming.

8) THE VILLAGE

It turns out they are all in some type of fake reenactment community in the middle of a national park with fables of monsters to keep the younger kids in check from exploring the surroundings.

7) SE7EN

Brad Pitt can't count. There's still two more deaths to come.

6) FIGHT CLUB

'The Narrator' (Edward Norton) is also really Tyler Durden (Brad Pitt), and has a knack for beating himself up ... and making soap while he should be sleeping. 

5) SAW

Haven't seen this one as the Saw movies are not my thing, but the ending goes something like this - the dead dude that's been in the room all along is really alive, and is the killer.

4) ACE VENTURA

Ok, be honest - who really knew that Lois Eindhorn was really a man and Ray Finkle in disguise? See, a worthy great movie twist.

3) THE MYSTERIOUS ISLAND OF BEAUTIFUL WOMEN

Oldish movie, but I reckon JJ ABrams saw this before he had the idea for Lost. Even has quite a sinister theme for a family movie. I won't spoil the twist about the leader of the island as you probably haven't seen it - but try to track it down!

2) EMPIRE STRIKES BACK

So big bad Darth is Luke's Old Man. No one in 1981 saw that one coming.

and, no surprises, Number 1 ...

1) It's got to be THE USUAL SUSPECTS

Who is Kaiser Soze? The ending is so phenomenal, it takes away from the rest of the movie, which is actually pretty good!

Worthy mentions:  Psycho (1960), Sixth Sense (1999), Planet of the Apes (1968), Memento (2000), The Others (2001) ...