Well, the strongest possible is "Great Maju Garzett". If you were to build a Traditional Format Deck that had 2 "Jinzo", 1 "Cyber-Stein", and 3 of all other Machine-Type monsters, and you were able to dump all but 1 card from your Extra Deck into your Graveyard (to include Machine-Type Fusion Monsters), then you'd get "Chimeratech Overdragon" with somewhere around 25000 ATK. Tribute that for "Great Maju Garzett", then power him up with "Scapegoat", "United We Stand", "Mage Power", "Mystic Plasma Zone", and 3 "Ballista of Rampart Smashing", then attack a face-down monster.
Realistically, though, probably "Five-Headed Dragon", with its 5000 ATK.