Char B1 was the best tank when France was invaded, as the current German panzers could not destroy it from the front. However, the German Blitzkrieg basically was a Do not stop moving until the enemy has fallen, and the German panzers were much faster than the B1, allowing them to circle it and hit it in its nice juicy side and back armor. One of the many reasons France was lost early WWII, along with stuff like the confusion caused by the Germans, and the fact that the only french tank that could keep up with the German advance was an infantry tank with a gun that fired mainly low velocity HE rounds, not suitable for destroying tanks.
The B1 wasn't, it was, to be blunt, the ideologies of an older time period come to reality. It was a cool tank, however tanks like S-35 serious surpassed it, and other German models. The French and BEF were more than capable of withstanding the German invasion and had in fact almost broken the German line of advance many times. Both the BEF and French military had quickly organized their own mixed formations of motorized infantry and armor, which seriously hindered German advance.
The main worry of the French wasn't in its military, but a return to the WW1 style of warfare. France had taken the most losses of the Western Allies in WW1, both in man power and in industry, it had a declining birthrate, was suffering from a very bad economy, which added up to a population that did just not want to fight. This lack of morale effected both troops and politicians, which wanted a very quick end in order to avoid heavy losses.
The B1-bis was the heaviest tank in 1940 and was the french "Tiger" on the West front. The best german tank, the Panzer IV Ausf. D or E was unable to destroy it. But I think that the best french tank was the cavalry tank Somua S-35. ^^