I'd say in a 1 on 1 fight, a German tank in 1940 would most likely lose to a British or French tank.

The main armament of the Panzer III (which was the designated "tank on tank" unit for the Wehrmacht), was the 37mm KwK 36 L/45. With the standard armor-piercing high-explosive "PanzerGranate 39" (PzGr 39) round, you could penetrate about 29 mm of armor from 500 meters.

The Panzer III's French opponent in 1940 would have been the SOMUA S35. The S35 is considered to be one of the best, if not the best, medium tank on the battlefield in 1940. The S35 has 47 mm of armor which means PzGr 39 rounds will bounce right off, and the S35's own 47mm SA 35 gun can penetrate 33 mm at 500 meters.

The armor on an early war Panzer III is no thicker than 30 mm anywhere on the tank, which means the 47mm can make Swiss cheese out of a Panzer III without the Panzer III being able to put a dent in the S35.

Plus, manufacturing difficulties meant that most of the tanks in the field in 1940 were Panzer Is and Panzer IIs, tanks that had machine guns (PzI) and a machine gun and a 20mm cannon (PzII) for armament and paper-thin armor.

The Renault B1 had the same 47mm as the S35, but also armor between 55 and 60mm thick, meaning a Panzer III had no chance to dent the armor before getting taken out.

British tanks such as the A13 were a bit easier for a Panzer to handle, with armor of up to 30mm but as thin as 6mm. However, the 40mm QF 2 pounder gun the A13 had could reach out and touch someone at 500m with 37mm of penetration, again going through a PzIII.

However, the big bad bitch of the British tank corps was the Matilda Mk. II, with the same 2 pounder gun and with 75-78mm of armor. During the Battle of Arras, a concentrated force of Matildas was only driven back when the Germans turned their 88mm anti aircraft guns earthwards and loaded them up with armor-piercing rounds.

The reason the German tanks got such a reputation as being invincible during the Battle of France was the doctrine used by the Wehrmacht in armored warfare. Each German tank was equipped with a radio, something not done with British and French tanks. This made command and control a hell of a lot easier for the German tank commanders. Plus, German armor doctrine mandated that tanks be used together as an armored spearhead, meant to race out in front of the ground pounders to seize bridges, supply depots, etc, in the hopes of cutting off an enemy's route of retreat and forming pockets, or "kessels," which turned into a battle of annihilation.

French and British tanks were usually used piecemeal and subordinate to infantry units and not really suited for coordinated offensive strikes. The Germans outnumbered the enemy and were able to destroy them with numbers instead of superior vehicles.

Sources

Henry, Chris (2004). British Anti-tank Artillery 1939-45 Pascal Danjou, 2004, L'Automitrailleuse de Combat SOMUA S 35 Panzer III & Its Variants (The Spielberger German Armor & Military Vehicles, Vol 3)

Years and years and years of getting my ass handed to me by French and British tanks in WW2OL