I would have included the Jeep/Willys, but they were more military than civilian when they made thier mark.
Here was my reasoning:
1. Model T
The first mass produced car in the U.S. which allowed middle income America to finally own a car.
2. Beetle
The most prolific car ever sold in the U.S. and still the easiest car to recognize from any distance. The versatility of the Beetle goes unchallenged today.
3. 427SC
The first U.S. made car to win the Wolrd Manufacturer's Racing Cup and still one of the fastest cars ever made. It epitomized what would later be called the "Muscle Car Era".
4. Original Pontiac GTO
The original muscle car which started the trend.
5. Original Chrysler 300
The first Hemi engine based car that proved luxury and speed were not mutually exclusive terms.
6. Original Corvette
America's first 2 seat sports car and the first American car to have a body not made of metal. I believe it is also is the longest consecutive production car built in America.
7. Original Thunderbird
Ford's answer to the Corvette, but was geared more to the luxury buyer rather than the sports car buyer.
8. Plymouth Superbird
This car was wrought in controversy. Taking the lead from what Ford did with the Cobra, Chrysler built just enough of these beasts to murder everyone at NASCAR.
9. Mustang
The first "Pony car". Initally, the car was not a sports car, but rather a sporty car offered at low costs. The demand was so great Ford could not build enough of them.
10. Camaro
Chevrolet was caught flat footed by Ford and quickly rushed this to production. Once in production the "Muscle Car Era" would never be the same. The competition between General Motors, Ford, AMC, and Chrysler was really brought to life once the Camaro debuted.
To be sure, there are other automobiles that made thier marks in our history and this list is mostly comprised of hot rods, which I have a partial affinity for.