The geysers.... oh what bad memories I have of that Colossus.
That was the first Colossus I was truly stuck on, and I stopped playing the game for like three months as a result. I finally went back and finished it, though, and boy was I glad I did. I didn't quite get the full impact of the game until my second playthrough, however, when I beat it without any significant breaks.
#9 will always be entrenched in my mind as *that* colossus that kept me from finishing the game.