Which principality was the strongest during the period of feudal fragmentation and why?

The Novgorod principality is the richest land of the Russian Middle Ages. Novgorod was isolated from the rest of the Russian principalities and was in no hurry to participate in general Russian affairs. Having escaped the Mongol invasion, Novgorod did not seek to pay tribute. Beekeeping, fishing and hunting for fur and sea animals were developed in Novgorod. Trade, crafts, and the development of trades developed successfully. Due to the availability of access to the Baltic Sea, Novgorodians traded with European states. Cloth, metal, weapons were exported from Novgorod to neighboring Russian principalities. Economic prosperity led to the formation of an independent boyar republic.