As much as I love the D.520, I'm voting my favorite being the Arsenal VG-33 series, and probably the best had it seen mass production. What is so unique about this design is that it called for the use of certain non strategic materials which could be cheaper and quicker to build than other fighters by normal standards at the time. It was also built under less than ideal circumstances because of competition from other aircraft manufacterers, all on the eve of war. I'm tempted to believe the designers/builders of the Arsenal VG series were able to see the dark clouds on the horizons because of these factors. As such, I would classify the Arenal VG-33 series as something resembling that of a "desperation" fighter design, one which could have made a difference had it seen combat on a wide scale. The Caudron C.714 could also be classified as a french "desperation" aircraft given it's construction.
Only 12-13 VG-33 examples were fully built and of these only 2 reached operational status. A few French publications I have mention that one of the two may have shot down 1, perhaps 2 German planes before hostilities ended, though there is some debate in the various French aviation forums about this. One Stuka and one Me-109 are the two aircraft often mentioned. I don't think anyone has concluded either way on this yet.
My other favorites are the D.520, Breguet 693 series, the MB.170-175 series.