The best :O moment in KH.

when Xigbar said he was half Xehanort

 

BBS: Eraqus' death or Aqua falling into the Realm of Darkness.
KH: Riku takes back the Keyblade at Hollow Bastion.
COM: Sora chooses to forget his adventures in Castle Oblivion.
Days: Riku defeats Roxas.
KHII: Finding out DiZ is Ansem the Wise.
coded: Deleting the first Data Sora.
3D: Sora fails to save Rinzler and Xemnas revealing that Nobodies can "grow" hearts.

 

**SPOILERS FOR DREAM DROP DISTANCE**

The biggest for me was when they finally revealed the connection of the orange/yellow eyes--with Xigbar and Saix revealed to be part of Xehanort--and basically the Organization's REAL purpose.

Also, Riku becoming a Keyblade Master while Sora did not AND Riku being Sora's Dream Eater during the test.

Lea getting his own Keyblade WAS a suprise, though I saw it coming (Yen Sid spoiled that a little early by saying something about a new Keyblade weilder's fiery personality or something, as I recall). Otherwise, I really would have been shocked.

Pretty much everything in Dream Drop Distance, actually xD

Figuring out that Xehanort was actually Terra was a big one for me too, though I'd been suspecting a connection due to the similar hair style. Also, that Castle Oblivion was the Land of Departure.

Really cool stuff. I could probably think of a ton of other moments (and will think of them the minute after I post this).

 

I'm putting these in spoilers in case a viewer hasn't played any of the following games yet.

KH1:

Spoiler

When Riku takes the keyblade from Sora.

KH2:

Spoiler

Finding out the "Ansem" you killed in the first game was just a heartless.

BBS:

Spoiler

Finding out Aqua created Castle Oblivion, and seeing Vanitas looks like Sora.

Days:

Spoiler

Finding out about a 14th Organization member and that she appears differently to different people.

Re:/CoM:

Spoiler

Seeing Axel killing off the Organization members. It made me question whose side he was on.

Re:/Coded

Spoiler

Seeing Pete and Malificent in the cyber world.

DDD:

Spoiler

When Young Xehanort used his Mysterious Figure moves in the middle of battle (and making a clone of himself). I kind of panicked when he did that since I couldn't handle him in BBS.