If we exclude F1 non-championship race wins it's down to Amon and Behra, with a nod to Mike Spence and Karl Kling.
If we include them then I would suggest:
-Olivier Gendebien, who won Le Mans four times when it was chock-full of the cream of Grand Prix racing;
-Roy Salvadori, who handily beat Jack Brabham in identical cars in 1958. Would have been a favourite for the '59 title, but he moved to Aston Martin...;
-Piers Courage, who won in the Tasman formula, which was as competitive as your average Grand Prix, and scored two second places for Frank Williams, a run of success that the team did not match until the Saudis filled their pockets with gold;
-Jean-Pierre Jarier, who would have won a couple of GPs for Shadow but for reliability, and would have won at Long Beach in 1983 had he not come across a wounded Rosberg at the wrong time.
There are other drivers who showed GP-winning pace but never really got the chance, such as Peter Arundell or Chris Irwin, and Jackie Lewis might have been a contender had he not retired to a Welsh farm before he reached his peak.
And for all the genius that was Fangio, back in Argentina there was another driver who could match him in the town-to-town rallies. He only had the one Grand Prix but picked up a couple of points from it, less than half-a-minute behind team-mate Gonzalez after over three hours of racing. Just how good could Oscar Galvez have been?