Archrival (noun)

A rival that is considered to be the most significant or formidable.

Origin:

From the words "arch" meaning extreme and "rival" meaning an opponent, literally meaning "most intense rival".

Examples:

1. His archrival in the competition had always been a tough opponent.
2. The two companies have been archrivals for decades.
3. The two scientists have been archrivals since their graduate school days.
4. He has an archrival in the race for the presidential nomination.
5. The team's archrival was responsible for ending their winning streak.