All of this thread is interesting, of course, but let's take a look at some numbers that really tell the tale, as to which car was truly the best selling car of all time. Some things to ponder here:
1) The number of cars on the road.
2) Yearly automobile production.
3) Percentage of annual sales achieved by the best-selling car in a given year.
1) By 1910, there were 458,500 automobiles on the road in the US, far more than in any other single country on the planet.
2) Ford Motor Company sold 32,053 Model T's that year. By 1912, and the opening of their Highland Park MI plant, Ford had a production capacity of 26,000 cars a month, or in excess of 300,000 cars a year, and by year's end, fully 75% of all cars on the road in the US were Fords. In 1914, Ford sold 308,313 Model T's, all produced in Highland Park. (GM very nearly went bankrupt in 1912, Ford announced the industry's first rebate --$50 to each buyer of a new '14 Model T if the company sold at least 300,000 cars that year, and Ford sent out a $50 check to each new 1914 T buyer, my grandfather got one of those checks). In 1915, the US automobile production to over 800,000. The number of cars in the US reached 2,000,000. In 1915, Willys-Overland was the 2nd largest US automobile company, producing 91,780 cars. In 1916, US automobile production passed the 1 million mark and Ford sold 734,811 Model T's. (by 1917, the total number of cars on US roads was 4.8 million, the rest of the World combined had just 720,000 cars.
3) In 1921, Ford held what was to be the largest share of the US market of any automaker since--some 61% of all cars built in the US were Model T's (GM had just 12%, and once more, was nearly driven into bankruptcy). By 1924, the production of the Model T was just shy of 2,000,000 cars, and held the highest market share of any car, all over the World, with more than 50% of all cars everywhere being Fords.
I would submit, that while other specific models of cars have surpassed the production figures above, as the worldwide production and sales of cars grew since 1924, no single model of automobile has ever achieved anything like the percentage of total production (in the mass production era which still goes forward) nor the market penetration and market share achieved by the Model T Ford. And all of this happened in a time when there were vastly more companies/makes of automobiles than at any time since.