Mark Donohue's Penske-Sunoco Camaro: The unfair advantage. Donohue recounted that the #15 and #16 cars, one at legal weight and one lightweight, had their numbers similarly shaped so they could be switched from vehicle to vehicle and the legal weight car would be sent through tech twice, once as #15 and again as #16
Bob Sharp Nissans: Especially Paul Newman's
BRE Datsuns and Nissans: Any of them. The Roadsters, 510s, and Z-cars were all awesome
Corvette C6.R: It dominated ALMS racing so much that it drove all the competitors away. Dodge, Aston Martin, and Saleen all dropped support for teams in the series. It won Le Mans a couples times too
Smokey Yunick's "7/8-scale" Chevelle: Another cheater car. Story goes that after being in tech for 11 hours at the 1968 Daytona 500 and refusing to pass it until 10 items were addressed, Smokey jumped into the car, which had been drained of fuel, started it up, said, "Make that 11," and drove the car out of the speedway and back to his shop