Greatest and Uppermost

Greatest adjective - Coming before all others in importance.

Uppermost is a synonym for greatest in position topic. In some cases you can use "Uppermost" instead an adjective "Greatest", when it comes to topics like top, highest.

Uppermost adjective - Being at a point or level higher than all others.

Greatest is a synonym for uppermost in position topic. You can use "Greatest" instead an adjective "Uppermost", if it concerns topics such as top, chief, predominant, highest.