Yes the Gilles Arnoux battle was incredible and probably the best second best scrap I've ever seen (the first being a 7s club race at Donington which had 6 leaders in the last 2 laps). Though as it was a scrap for 2nd place and was 'only' a Grand Prix I'm not so sure if it's the greatest IMO.
Thinking Nuvolari's because of the political significance of beating the Silver Arrows in an obscure car at the German GP in front of Hitler as well as the greatness of the drive. Le Mans 69 has everything for me, cars that shouldn't have been winnig after the 917s packed up cars fighting in the last laps of a 24 hour race, Ickx's protest of walking to the car on the start and it's on video, not sure if you can download it though.