I improvised the entire medley; I had a bit of help from ichigo's ( http://www.ichigos.com ) with Hikari and I wrote some sheets for Hallow Bastion and Darkness of the Unknown myself.
I would have included a lot more songs, since the soundtrack is just plainly incredible, but 20's a good number and 19:43mins is already pushing it. (Sorry to you "Passion" fans) KH / KH2 music (c) Disney, Hikaru Utada, Yoko Shimomura, Squaresoft, Square-Enix, other people; This is Halloween (c) Danny Elfman. Please don't sue me. Note: this is my first (recorded) piano piece, so please flame politely.
EDIT: Um. for some reason youtube didn't like my video and decided to meddle with the audio and video synchronization. *shrugs*
EDIT 2012: Video synced. Only took 5 years, YouTube :)