Not necessarily the GOAT, but more who is the "face" or biggest household name of that particular sport.

Association Football/Soccer:

Baseball:

Basketball:

Boxing:

Cricket:

Cycling:

Darts:

Formula One:

Golf:

Gridiron/American Football:

IndyCar:

Hockey:

Horse Racing:

MMA:

NASCAR:

Professional Wrestling

Rugby:

Skateboarding:

Snooker:

Swimming:

Male Tennis:

Female Tennis:

Track and Field: