By Hawk-Eye (alias Y. Michelet) : the most successful French fighter?

In order to answer your question in a short and direct manner - I think you meant what AC was best independently of pilots, experience, circumstances etc. - obviously it was the Dewoitine D.520. Yes Juha it was REALLY good. Look at Danel and Cuny's book "Le Dewoitine D.520" very closely! It was much faster and climbed better than the Curtiss, and had one deadly cannon (Werner Mölders confirmed this when he reported how he was shot out of the sky by a 520, which he mistook for a "Morane").

The Curtiss was excellent actually but IIRC it was designed several years earlier than the D.520. France had already massively ordered the P-36's successor, the P-40, which was quite good by 1940 standards. Deliveries would have started 1940.

So the different French fighter types didn't quite belong to the same generation. It is even visible on the D.520, the horizontal stabilizer of which had NO struts contrary to both MS 406 and Me 109 E. CLearly aircraft design was making progress.

The D.520 was a more recent design and it was the best French fighter. More than 400 had been produced when the French Campaign ended (427 it seems) and several hundred were left, especially in North Africa, where they had been sent by Government order.
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Too bad we can't take into account the French fighter types which were "in the pipe" already but couldn't reach the first-line units in time, or just in time but too late (this was the case of the much-improved Bloch 155 : armored windshield, two belt-fed cannon, more powerful engine etc.). If the French territory could have been preserved like the British territory was, these superlative fighters would have cut the Luftwaffe to pieces already 1940 : Arsenal VG-33, 36 and 39 (I have no doubt that they would have proved themselves in combat), Dewoitine D.523, 524 and 551 (prototype flew at a speed of 702 km/h in 1939) and others. It has to be stressed that very fast, simple mass-production had already been planned and organised, especially at Dewoitine's, which was in a position to turn out D.520s, 523s and 524s (they got more and more powerful engines up to 1,200 ch and were faster and faster) at an "astounding rate", as a British author wrote about the 520 production. The D.520 had an engine power of 930 ch.