If you are going for PURE damage...FFVII "Knights of the Round" summon rocks them all. Even magnus sisters from FFX would not compare to the 9 brothers. Fenir from FFXI was awsome, and probably the toughest summon to recieve from any of the FF games. As far as the coolest...Bahamut Zero (that was the one that attacks from space right) from FFVII. Followed close by the Bahamut from FFX.