The Game Guy
UPDATE (6:43) I'm went back and checked out exaclty how Little Mac's KO move worked. He has a power meter that rests above his percentage. Connecting with attacks and getting hit by enemies will fill his power meter. When the meter is full, he can land a powerful auto-KO shot.
His final smash turns him into GigaMac. He becomes a super human version of Mac that packs an emormous punch.