arch rival

Translations
заклятый соперник
The two companies have been arch rivals for decades.
Эти две компании были заклятыми соперниками на протяжении десятилетий.
Definitions
An arch rival is a person or entity that is considered to be a primary opponent or competitor, often characterized by intense rivalry and competition.
Words
arch

adjective
заклятый
rival

noun
соперник