@craig-o, I think you’ve hit the nail on the head there, but you haven’t said the answer. The four closest finishes in F1 are, in reverse order:
de Angelis beats Rosberg by 0.050s at 1982 Austrian Grand Prix
Senna beats Mansell by 0.014s at 1986 Spanish Grand Prix
Barrichello beats Schumacher by 0.011s at 1992 US Grand Prix
Gethin beats Peterson by 0.010s at 1971 Italian Grand Prix
So the answer must be Ronnie Peterson. Francois Cevert finished third in that race, just a further 0.080s behind Peterson.