* μέγιστος (mégistos): This is the superlative form of μέγας (mégas), meaning "great" or "large." It's a good general translation for "greatest" in terms of size, magnitude, or importance.
* πρῶτος (prôtos): This means "first" but can also imply "greatest" or "most important" in certain contexts, especially when referring to rank or precedence.
* You might also use a phrase like ὁ μέγιστος (ho mégistos), which translates to "the greatest," adding the definite article for emphasis.
The best choice will depend on what is being described as "greatest."