This question is something of a companion to this one. I am interested in finding out who the most influential Sith (or other Force entity) was in the Star Wars universe? Influence is not the same thing as raw power - regardless of personal abilities, which individual had the broadest and longest overall effect on the Galaxy?
closed as primarily opinion-based by phantom42, Jason Baker, Rand al'Thor♦, Adele C, TGnat Aug 16 '15 at 23:14
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"The Ones" (http://starwars.wikia.com/wiki/The_Ones) were the most powerful force wielders ever. "The Father" (http://starwars.wikia.com/wiki/Father_(Mortis)) in additional to be the most powerful in the force was the most influential.
"The Father" kept "The Son" and "The Daughter" at bay, and made sure there was balance in the force.
"The Father" also made sure the that Abeloth didn't destroy or ruin the universe.
Entire religions were spawned after "The Ones."
They've also been around the longest going back to 1,000,000 BBY.
In the Star Wars Universe nobody has influenced the force, or the universe more than "The Father" as he kept things in order for all that time and had a huge responsibility, without him things would have fell apart.
Since you included "force entity" in your question, this is a clear cut answer.
The Clone Wars "Overlords":
The Father: You cannot imagine what pain it is to have such love for your children and realize that they could tear the very fabric of our universe.
AS: I don't understand.
The Father: It is only here that I can control them A family in balance. The light and the dark. Day with night. Destruction replaced by creation.
AS: Then why reveal yourselves to us? There are some who would like to exploit our power. The sith are but one.
The Father: Too much dark or light would be the undoing of life as you understand it.
So the guy keeping "life as you understand it" together and holding the force in the balance for over a million years is definitely the most influential force wielder ever.
If you're going by just the movies, then "Darth Vader" as he was the most hands on and the stories were most centered around him and his character. You could also choose Palpatine as Richard suggests. Palpatine was a mastermind strategist.
But there wasn't anybody in the series that influenced the universe and for as long a period a time as "The Father" if you're including all force wielders.
It's hard to gauge who was the most influential Sith without a clearer definition of exactly what influence is.
Darth Bane, born under the name of Dessel, was the Sith'ari and the Dark Lord of the Sith responsible for creating the Rule of Two.
Another contender could be Darth Andeddu, he is beleived to be the first Sith Lord to prefix his name with the title: Darth.
Darth Andeddu, also referred to as the Immortal God-King of Prakith, was an ancient humanoid male who reigned as Dark Lord of the Sith, and was dubiously thought of as the forebear of the "Darth" title.