The answer is CLEARLY Mr. Mayweather.
My proof?
"Twilight."
SPOILERS if you haven't seen it, for the rest of this post.
Remember the battle with the two Xindi ships?
Mayweather died, so he was no longer at the helm. This allowed T'Pol to take over and ram the Xindi ships together, thereby damaging Enterprise.
Damaged, the ship couldn't get to the weapon in time to stop it from reaching and destroying Earth. Then, the human race was hunted nearly to extinction.
If Mayweather had still been around, Earth wouldn't have been destroyed. T'Pol wouldn't have taken the helm from him, knowing it would be logical to have the better pilot at the helm. And even if T'Pol had ordered him to ram the ships, Mayweather's a better pilot, and may have been able to do so with less damage to Enterprise.
Ensign Travis Mayweather is the key to Earth's very survival, and therefore the most important character on Enterprise, and indeed in all of Trek history. You owe him respect.