Not that I would ever doubt the Simpsons.... However, the mass of the brain is related to body mass by a ration of about 1/50 http://hypertextbook.com/facts/2003/RachelScottRosenbluth.shtml
Curly Howard was the heaviest stooge by far - even suffering from weight related problems later in life. So my assumption is that Curly had the heaviest brain.