Chris Amon is the classic, he was considered by his contemporaries to one of the top drivers (his contemporaries being Dan Gurney, Jack Brabham, Jim Clark, Jackie Stewart etc!).
Stefan Bellof was one of the all time great talents, he truly was on a par with Senna and Prost in terms of pure speed. He was the best not to win a Grand Prix for me.
Other honorable mentions are Ricardo Rodriguez, Tom Pryce, Vic Elford (who was and still is one of the best all rounders ever seen in 4 wheeled motorsport) and I honestly think of the last 20 or so years, Nico Rosberg is probably the best to have not yet won a race.