What is the name of the large marble in a game of marbles?

it's called a godfather - sorry it's not called this - see the following

Various names refer to the marbles' size. Any marble larger than the majority may be termed a boulder, masher, popper, shooter, taw, bumbo, bumboozer, bowler, tonk, tronk, godfather, tom bowler, giant. A marble smaller than the majority is a peawee or mini. A grandfather is the largest marble, the size of a pool table ball or tennis ball.