an arch enemy
main enemy, used for emphasis
arch enemy
someone's main enemy

[TahlilGaran] Dictionary of Contemporary English ▲

enemy someone who does not like you because you have had a big disagreement with them in the past or someone who opposes you in business or politics:
If that’s how he treats his friends, I’d hate to be his enemy.
In business, it’s best not to make too many enemies.
adversary formal an enemy:
When he retired, he was replaced by his old adversary.
He made the fatal mistake of underestimating his adversary.
foe literary an enemy:
A guard called out ‘Who goes there – friend or foe?’
his former foes
arch enemy someone’s main enemy:
In the movie, Batman goes into battle with his arch enemy the Joker.
opponent someone you are competing against, for example in a sports game, competition, or election:
You win the game if you are left with more cards than your opponent.
He is admired even by his political opponents.
nemesis written an enemy or opponent that is impossible to defeat – a rather formal use:
In the final, he met his old nemesis, Rafael Nadal.

[TahlilGaran] English Thesaurus ▲