Model T.
The first cheap mass produced car.(well, the second, but the first to have a huge impact on society.) It formed the bulk of american traffic for the three decades after its introduction. Although production ended in 1927, many were still around as late as the 1940s. The first american sedan, it was also the first SUV. Many drivers took them to places that the T wasn't even supposed to go. Also, a T was turned into the first snowmobile. Everyone is touting the Corvette because of its racing heritage. The first T and the First corvette weren't fast by their time's standard. And, while a corvette in 1965 was considered fast, a 1925 T was considered pokey when it was new. But, the T never really changed. Sure, it got another carb, and some new bodystyles, but it was largely the same. And while it was a slow car, it had lots of sporting potential.
What did louis Chevrolet do when he left his company in 1916?(or was it 1912?) He went and hot rodded Model Ts and their engines. An average joe could get a boat tail body and place it on his stock T chassis. The same run of the mill racer could get a Chevrolet OHV head for his 4 pot.
Louis built his brother Gaston a car powered by a Ford block that won the indy 500. Hot rodding owes its entire existence to the T. All the first rods were Ts or T based. Why? cars were out before the T, why not rod them? But, if a 1912 mercedes cost more than the average worker's salary, then why would someone want to hack the fenders off to make the car go fast? Hot rodding started because people wanted to go fast but couldn't afford the faster cars. putting another carb or adding a new head and taking the fenders off was a way for the average joe to get a piece of the excitement that only the rich or well connected could get at that time. racing hopped up model Ts at small tracks around the country brought racing to people who couldn't get out to new york or Indianapolis to see what it was all about.
Plus, they were, and are, really durable. many are running around today. Sure, there might have been a sandblast, patch, and repaint, but some of the Ts one can see puttering about car shows and local streets on weekends are almost 100 years old. A patch and paintjob can't begin to fix some of the newer cars. Many hotrods are using many T components. the fact that many T bodies can sit in a field for 50 years without rusting together is amazing. Sure, they require lots and lots of bodywork, but the 1985 Toyota Celica down the street that is 70 years younger is just as rusted.
It's the spiritual grandfather of the Beetle and Corolla: purely utilitarian and very reliable. But, without many drastic modifications, they could all be made to go very fast. Plus, they are all extremely simple to work on. The average person could perform routine maintenance on his or her T without much assistance.
From an importance standpoint, the T wins by a landslide. But, the Corvette was intended as a sportscar. In that category, it excels. But, the T was just intended to replace the horse and wagon. It excelled in that, but also transformed the nation and did many things it was never even intended to do. What do you think Henry would have thought when he found out that cars using T frames and bodies were doing 180+ at Bonneville?
Can you believe the moron in the picture actually took out the IRS when he first got it? And then the threw the parts out!! He had to have someone fab up an approximation of the (now missing) documentation. 
