KV-1 Model 1941. Only some 205 of the Soviet tanks used at Kursk were classed as heavies, Kursk marking the swansong of the KV-1. Many were destroyed at long range by the guns of the Tigers and Panthers.
In 1941 I recall reading that one of these monsters stopped AGN from going ahead for several days even if the germans used heavy artillery for shooting in straight line aiming through the barrel. I don´t recall if the crew abandoned the tank or the German engineers attacked the KV but it took at least a couple of days before the Germans could continue attacking.
Both were... problematic... on initial encounter. ww2f.com/threads/kv-2.9843/#post-156765 1: Rokossovsky: The KV tanks literally stunned the enemy, they withstood the fire of every type of gun the German tanks were armed with, but what a sight they were returning from combat. Their armour pockmarked all over and the barrels sometimes pierced. 2: 1st Panzer Division: Our companies opened fire from 700 metres. We got closer and closer... soon we were only 50-100 metres from each other. A fantastic engagement opened up - without any German progress. The Soviet tanks continued their advance and our ap projectiles simply bounced off. The soviet tanks withstood point-blank fire from both our 50mm and 75mm guns. A KV2 was hit more than 70 times and not a single round penetrated. A very few soviet tanks were immobilized and eventually destroyed as we managed to shoot off their tracks, brought up artillery to hammer them at close range and then attacked them on foot with satchel charges.
The story of the KV-2: The Russian KV Tank and Its Crew That Stopped The German Advance for a Full Day