I think the best individual Standardbred Breeder of the modern era would be Bill Shehan.
This piece is a bit dated with the horses it mentions, many more have been added since, but the foundation mares/familes that Shehan bred have been a massive influence on the Standardbred breed.

WILLIAM R. SHEHAN - (1929-1991)
William Shehan, a native of Ann Arbor, Mich., was born October 1, 1929.
He was a graduate of the University of Michigan, and for almost 20 years maintained a corporate law practice in New York City, and specialised in international finance.
While in college, Shehan rubbed horses for fellow Immortal Sanders Russell. A quick learner, Shehan developed his own ideas on the kind of gait it took to go fast and he tried to develop that gait within his own band of broodmares.
He was first listed as a breeder by the United States Trotting Association in 1955, and bred Fleeting Star. Adora, Golden Miss, K Nora, and Lover’s Walk, all very important foundation mares in Shehan’s band.
Initially breeding and selling yearlings under this own name, in 1977 Shehan developed Clermont Farm, a 560 acre Standardbred nursery in the Bluegrass State.
Bill Shehan bred expressly for the traits of good gait and physical type. His object was to breed away from any faults. He was also successful in bringing to life several branches of maternal families who have produced top performers -- generation after generation.
Adora was the dam of Adora’s Dream and K Nora. K Nora is the grandam of Three Diamonds, the dam of Life Sign. K Nora is also the dam of Angel Hair, who is the dam of Naughty But Nice and the grandam of Leah Almahurst, the dam of Western Ideal. The family of Golden Miss lists Real Desire, Grinfromeartoear Royalflush Hanover, all winners of over $1 million, and Rustler Hanover, a winner of $971,638.
Lovester was the result of breeding Lover’s Walk to Speedster. This Grand Circuit stakeswinning filly is the dam of Rosemary, voted the 3-Year-Old Filly Trotter of 1978, and who is the grandam of Royal Prestige. Shehan’s skills also link him to Self Possessed, Continentalvictory, and Act of Grace.
In total, Sheehan was the breeder of 300 Standardbreds, including the Little Brown Jug winner Strike Out, the Yonkers Trot victor Tamerlane, the world champion Lightning Strikes, the multiple stakeswinner Cavalcade, and Shifting Scene, Diamond Exchange, Hilarion, Justin Passing, and Short Stop.
The formation of the Kentucky Standardbred Sales Company in 1980 was the brainchild of Shehan, Tom Crouch of Kentuckiana Farms, and several other yearling consignors.
This Standardbred sales company produces one of the highest sale averages in North America, and boasts of having sold the highest priced yearling ever offered at public auction, Experience Victory, who sold for $650,000 in 1999.
Experience Victory’s grandam, Katie Almahurst, was owned as a yearling by Shehan.
Many of the fillies bred and sold by Shehan went on to exceptional broodmare careers at major farms, and their offspring continued to produce superior yearlings each year for the major sales.
Shehan passed away in 1991, having left his mark on the Standardbred industry as one of the great modern-day breeders.