This is one of those topics that gives a different answer depending on how you measure it.
The best-selling 'single model' is NOT the Volkswagen Beetle, much as we might want it. It is the Toyota Corolla, with 35,000,000 sales over nine generations since 1966. Second is the Ford F-series, with 32,000,000 over eleven generations since 1948.
Third is the VW Golf, with 26,000,000 in five generations (the 6th went on sale recently) since 1974. The Golf overtook the Beetle in 2002.
Fourth is the Beetle, with 21,529,464 from 1938 to 2003. Now people have often said that the Corolla, Ford F-series and Golf have been redesigned a number of times and aren't the same car, while the Beetle has had the 'same body and design' since 1938. Unfortunately that isn't true. The Superbug was a major redesign, with different body, suspension and name, and it was sold alongside the torsion bar Beetle. If you count the Superbug as a 'Beetle', then you have to count the Golf 1-6 as the same car too. Nowadays people only count 'nameplates', whether or not the vehicle has been redesigned. See
http://en.wikipedia.org/wiki/List_of_bestselling_vehicle_nameplates
In any case, the Ford Escort has now passed 20,000,000 and will pass the Beetle in the next year or two. The Fiat 124 isn't far behind, and even the VW Passat has now passed 15,000,000.
The good news is that VW does not count other cars built on the Golf platform, such as Jettas, as Golfs even though they are actually Golfs with boots. You could add 26,000,000 Golfs and 6,600,000 Jettas to get 32,600,000 'VW Golfs' - getting close to top spot again.