When you're the captain of a ship, you need to be able to reply on those under your command. Though every member of the crew is vital in their own way, there is perhaps none more crucial to the running of your ship than your first officer. The clue is in the name - they are the highest ranking official, outside of the commander themselves, and are responsible for ensuring the smooth operations.
History has graced us with some fabulous examples here. We have Spock, who effectively defined what it means to be the Number One. We have William T.
Riker, the trombone enthusiast and fiercely loyal Number One to Picard. Kira Nerys served as executive officer under Sisko, though without the Starfleet uniform to match. On Deep Space Nine, she was a critical component not simply in running the station, but in ensuring the peaceful cooperation between Starfleet and Bajor.
There are dozens of examples of fine, fine officers - all of whom could top a ranking of greatest contributors to history. This is not that list. This is, simply put, the rest.
10. Will Decker In a way, this is low hanging fruit. Decker, son of the late Commodore, only appears in The Motion Picture where we, the audience, are treated to a series of the man's demotions.
Decker, in fairness to him, did very little to deserve these things. Now, why is he here? If we take the pros first - he knows the Enterprise far better than Admiral Kirk at this stage, even saving the ship with his knowledge in the wormhole - then there .