* ἀνίκητος (anīkētos): This is probably the closest single word. It literally means "unconquered" or "invincible," emphasizing the inability to be defeated in battle or competition.
* ἀνυπέρβλητος (anupérblētos): This means "insurmountable" or "un surpassed," suggesting something that cannot be overcome or exceeded. This works well if "unbeatable" refers to a superior skill or quality that's difficult to match.
* A phrase might be more accurate than a single word, for example: ἄτρωτος καὶ ἀνίκητος (átrōtos kai anīkētos) meaning "unwounded and unconquered".
The best choice will depend on the specific situation where you want to use the word. If you can provide more context, I can help you choose the most appropriate translation.