adversary — Your adversary is someone you are competing with, or arguing or fighting against.
challenger — A challenger is someone who competes with you for a position or title that you already have, for example being a sports champion or a political leader.
competition — Competition is a situation in which two or more people or groups are trying to get something which not everyone can have.
contestant — A contestant in a competition or quiz is a person who takes part in it.
favourite — a person or thing regarded with special favor or preference: That song is an old favorite of mine.
favorite — a person or thing regarded with special favor or preference: That song is an old favorite of mine.
opponent — a person who is on an opposing side in a game, contest, controversy, or the like; adversary.
rival — a person who is competing for the same object or goal as another, or who tries to equal or outdo another; competitor.
antagonist — Your antagonist is your opponent or enemy.
opposition — the action of opposing, resisting, or combating.
dark horse — If you describe someone as a dark horse, you mean that people know very little about them, although they may have recently had success or may be about to have success.
