Cannon A-series.

Now wait, don't lynch me yet. Hear me out! Much like Ford and his Model T, it's not about marketting or sales. It's that this product worked, worked WELL, and was cheap, and easily mass-produced. The Model T pushed use of the personal automobile to unimaginable limits at the time.

The Cannon A-series probably did more for spreading the joy of photography to the masses than Leicas ever did simply because they made (and sold) over 6 million of the A-series. It had professional-level attachments and quality yet was within the grasp of average people. Regardless of your personal preferences on ergonomics or layout (Olympus, Nikon, etc) I have to say the A-series was one of the most important 35mm cameras of all times.

In retrospect, of course.