What is the Latin word of the word unbeatable?

Invictus es when addressing a man.

Invicta es when addressing a woman.

Invicti estis when addressing a group containing at least one man (yeah, I know).

Invictae estis when addressing a group entirely of women.

The Word invictus is, literally, "unconquered," but it was used by ancient Roman writers to mean "invincible" as well.

Another possibility is to substitute insuperabilis(singular, either gender) or insuperabiles (plural, either gender) for whichever form of invictus. This is from the verb superare, meaning "to surpass, surmount, Excel" or in military use "to conquer."