Classe Tous Risques is pretty terrific but I'm also a big fan of Belmondo's three Melville films with Le Doulos being the fave. He also played a priest in Leon Morin but the one I have always found intriguing is Magnet of Doom (L'Aîné des Ferchaux), which should be run through the Noir-O-Meter.
In the wake of his death, I've tried to watch That Man From Rio, something of a Bond spoof. Not warming up to it yet, but it does hint at what Belmondo was capable of as a stunt man. He reportedly did most of his own stunts until he was into his 70s.