Six cylinder rivals.
Of course there were plenty of rivals for the six cylinder Mercedes cars in the 1960s. The six most important rivals are listed here below. The manufacturers of these cars didn't reach the same high production numbers as Mercedes did, though the Mercedes six cylinder cars were more expensive than the other ones. Looking at the 'home-market', the Opel Kapitän came in closests when it came to real competition. Also made in Germany, the Borgward P100 even had a similar air-suspension system as the 300SE but the car was never a real competitor for the top model of Mercedes-Benz.

The four cylinder rivals.
The other end of the Heckflosse line, the four cylinder cars, had it easier when it came to competitors. In 1961, when the 190 and 190 came out, they created their own class of vehicles. Even the old Pontons 180, 180D, 190 and 190D had little to fear from other cars. There were a few other cars who could be seen as rivals for the four cylinder Mercedes cars, though at the time they never really threatened the small Heckflossen. The BMW 1500 and later on the 1800 could be considered as a threat were it not that these BMW's appealed more to the sporting driver. The Peugeot 404 did show similar styling like the tail fins designed in this case by Pinin Farina. It was also the only small car that could be supplied with a Diesel engine. The Citroën was technically more advanced than the four cylinders but its bad reputation as a import-car and the small number of Citroën-dealers in Germany meant that it never became a success.