Biggest seller of all time.
Toyota Corolla across various generations... Or the Air cooled VW Beetle which went largely unchanged and can be considered a singlel model generation. All depends on what criteria you have in mind.
Beetle without a doubt IMO
Ford Model T, followed By the Ford F150
Toyota Corolla > 32.2 million units since 1966 (9 generations) VW Beetle > 21.5 Million of the air cooled design produced in some fashion from 38 till 2003 Model T, just over 15 million F150 , I can't get the damned total production numbers since Ford likes to play with statistics to say they are the best selling trucks, while truck class to truck class the combine GM and Chevy sales have dominated with very few exceptions, but are counted separately while Ford counts the sales of ALL F-series trucks to be right about 30 million since 1948... F150 didn't make it till the 6th of the 12 generations of Ford F-series. (Give you an idea of how screwy the numbers are on trucks, in 2006 the F series sold 901,463 and was declared the best selling truck of the year. However, the combined sales of GM and Chevy was 935,468, but they divvy their sales number out through the branches....) There, hopefully that ends the confusion...
9 generations of Corollas though... The beetles didn't really change that much