Good topic, here’s a list of drivers who have won just 1 GP (excluding Indy 500 winners):
Maldonado, Rosberg, Kovalainen, Kubica, Trulli, Panis, Alesi, Nannini, Nilsson, Brambilla, Mass, Pace, Beltoise, Cevert, Gethin, Scarfiotti, Ginther, Bandini, Ireland, Baghetti, Bonnier, Musso, Taruffi and Fagioli.
For me, it’s Francois Cevert by a loooong way. Other notable mentions are Nico Rosberg, Robert Kubica, Jean Alesi and Luigi Musso.
If I may go one step further and add drivers who haven’t won a race in their career, then I would have a tough time choosing between Cevert and Stefan Bellof.