Monaco GP 1934. After a race long battle between Nuvolari (Alfa Romeo) and Varzi(Bugatti) with some 17 lead changes, Nuvolari led by 4 seconds at the end of lap 90. To quote David Hodges's history of the Monaco GP:
... Varzi, however, was far from beaten and pulled up, level with Nuvolari as the 98th lap ended, momentarily ahead as the two started their 99th lap. On the climb to the Casino for the last time, Varzi held onto third, stretching his engine to the limit and risking all at over 7000 r.p.m., to take the lead. Nuvolari held him but briefly, for a piston failed as he over-revved and hot oil sprayed onto hot exhaust. So as Varzi alone for the first timein the race, took th flag, a yellow-shirted figure jumped from his car asit lost momentum down from the tunnel and started to push. Worried about a fire which did not exist, a mechanic helped and Nuvolari was thus disqualified.
So intended one of the mostintensely fought duels in the history of the sport.
Reims 1961 when Baghetti (Ferrari) just held off Dan Gurney (Porsche) to win by 0.1 sec
Le Mans 1966. Ford's attempt at a dead heat went wrong and unexpectedly gave victory to Chris Amon and Bruce McLaren form Ken Miles and Denny Hulme
Le Mans 1969. Jacky Ickx in the Gt40 just held off Hans Hermann in the Porsche 908 to win by 1.9 seconds for the closest finish at Le Mans
I believe the last 3 are on You-Tube, but I doubt if film exists of the Nuvolari/Varzi duel.