When you said, "The Big Boys", first one I thought of was Steve Crist (for teaching us how to play multirace wagers); and BTW, for teaching how important it is to actually form an educated opinion on a race, unclouded by emotion. Going back a bit, Beyer stands by himself.
Love him or hate him: D. Wayne Lucas for bringing in big owners, in a big way, and changing the fabric of racing forever as a result.
Allen Jerkins, for being willing to take on Chuck Simon
Arthur "Bull" Hancock for bringing good European bloodlines to the US, and improving our stock. Leslie Combs for pushing syndication and bringing money and "outside rich people" into the sport.
Descendents of many owners of the last century, who althought fewer in number, still "breed to race" - and gave us Secretariat, Ruffian, etc.
Coolmore and the Sheiks for buying up and taking the best American bloodstock away, but while doing so infusing a tremendous amount of cash that trickled down throughout the industry. If that was good or bad, nearly 20 years later, it's still hard for me to determine. It certainly seemed to spawn a profusion of "bloodstock agent as a career".