Toughest little guy ever... no contest, it's Tattoo from Fantasy Island. That sneaky little #&*!@%@ lured you in with his cheery smile and chubby little cheeks. Then, once you got in close he showed who was the "boss" by delivering painful blows to his opponents groin..
Just remembered another memorable little guy: Mitch Wilson, the 5'9 guy the Penguins tried out as Lemieux's bodyguard for a few games in 86-87. Mario apparently wasn't impressed by the idea of having a guy a half-foot shorter than him protecting him.