Which year was the fastest Porsche 912?

INone! no, really - 'd be interested to know why you're asking. Do you want to buy one? As Paul says, the differences are minimal. If you are just curious, see his answer. If you're thinking of buying one, consider that they are 30-40 years old. If you want speed or performance, and a 912 (usually mutually exclusive, depending on how broad you like to define things), then look for the one with the mods. Look for one with upgraded suspension, bigger brakes, engine upgrades, exhaust, etc. Not that these are usually heavily modified, but that would make the difference between two examples, vs original stock performance. For comparable stock original examples, consider which you prefer (color, condition, price, styling, wheels, etc). Good Luck - Jack