Arch enemies are two individuals or groups who have intense, long-standing animosity or hatred towards each other. The word "arch" signifies the extent and intensity of their enmity. The antonym of arch enemies would be "close allies" or "best friends." These two terms signifying a strong, positive relationship between individuals or groups rather than the negative, antagonistic one seen with arch enemies. Close allies or best friends signify mutual respect, support, and trust. They stand by each other's side through thick and thin, and their relationship is characterized by positivity and growth. While arch enemies see each other as enemies to defeat, close allies or best friends support each other and help each other succeed.