Honestly, without trying to be trite, I have to go with Ms. Buffy Summers.
Granted there could have been other slayers that may have been able to do the great things she did, although I do not think any of them could have aligned the Scoobs like she did.
And even though she had the help of Xander, it is likely that no other slayer could have taken out the Master. Because don't you think many other slayers would have tried to take out the Master in all the time that he was on the earth because he was the most notorious of all vampires?
Then you have the fact that she was what inspired Angel to do good. As soon as he saw her, he wanted to help her in her fight against darkness. Without ever meeting her, he might still be eating rats in an alley and smelling bad.
Not to mention that if she had never slept with Angel and caused him to lose his soul, then Willow might have never delved into the black arts. This was the key event that inspired her interest.
Then you have the many other countless times she saved the world.
Without Willow or Darla, I think the world would still be in somewhat a somewhat decent state. Because Buffy could have continued the fight without Angel and in a world without Willow, who knows that Buffy wouldn't have died in the Gift. After all, Willow's lover Tara was the one who gave Glory the insight that the key was Dawn. Without that one incident, Buffy may have never needed to jump off the tower.
But imagine if Buffy wasn't activated by India's death, then most likely the slayer activated by India would not have came to Sunnydale. Not every slayer guarded the hellmouth. Hell, the Watcher's Council didn't even know about it until Buffy and Giles informed them of this.
Without Buffy, the events of "The Wish" would most probably be in effect.
But I'm not saying she does everything perfectly, because lets face it, many people died because she lived. If she would have remained in Heaven, then the First Evil wouldn't have killed off so many potentials and members of the Watcher's Council.
So I guess it is really just the way you see it.