Roland Ratzenberger, Imola, 1994. Right at the start of his F1 career with so much promise. Ayrton Senna was bad but for me personally his death was less shocking as he had fulfilled his potential in F1 and was never going to be forgotten. Ratzenberger has been forgotten from the events of that terrible weekend in April 1994.
I wouldn't say he's been forgotten. More like his death has been overshadowed by that of Ayrton's, and understandably so. It's a crying shame, but it was always going to be that way. 
I wouldn't say that it is understandable at all! Someones death should not be overshaddowed or forgotten. Just because they were not as famous does not make their death any less tragic or shocking.
Sort of along the same line I feel that a real crying shame was the death of Ricardo Rodriguez at the 1962 Mexican Grand Prix. He was 20 at the time, nearly won on debut for Ferrari. The guy was a freak. Everything he touched turned to gold. He won cycling championships, then motorcycling championships throughout his teens.
He finished 2nd at Le Mans in 1960 at the age of only 18. This is what Wikipedia says of his short career with Ferrari:
Rodríguez was given a guest drive by Ferrari for the 1961 Italian Grand Prix, qualifying a surprise second and becoming the youngest driver in history to start from front row. In the race he exchanged the lead with Phil Hill and Richie Ginther many times, until a fuel pump failure ended his race. 1962 saw a full works drive with Ferrari, who used him sparingly considering his age and rough edges. Whenever used, Rodríguez shone, taking second at the Pau Grand Prix, fourth at the Belgian Grand Prix and sixth at the German Grand Prix in a tough year in Formula One for Ferrari. He also won the Targa Florio with Olivier Gendebien and Willy Mairesse.
If it weren't for bad luck, time and unforeseen occurrences, this kid could have been a legend.
