What is a 'first enemy' called?
As per Wikipedia, an archenemy, archfoe, archvillain, or nemesis is the principal enemy of someone or something. In vernacular English, archenemy and nemesis are the two from the four above that are actually used commonly.
There's no single English word to describe the single most important enemy in which case you need to combine two words:
- Primary Adversary
- Main Opponent
- Sole Archenemy or Sole Nemesis
Note to the OP: While archenemy has no friendly connotation, nemesis can also be used to refer to friendly rivalry.
The idiomatic opposite of Best Friend is Worst Enemy.
You are probably referring to an arch-enemy (archenemy):
- a principal enemy
Examples of arch-enemy:
The two politicians were archenemies.
The country went to war with its archenemy.
Source:http://www.merriam-webster.com/dictionary/archenemy