The boss at the end of FF8 was the best for me. It was an epic battle, literally had only Squall and Rinoa standing at the end. The bosses attacks where deadly, one where she could hit the whole parties HP down to 1, and if I remember, one where she could knock a character out of the battle.
But it wasn't the most difficult, that goes to the Dark Aeons in FF10, with attacks that do 99999 damage, they where almost impossable to kill.
