The theory that Plagueis used his power to cause the virgin birth of Anakin Skywalker is just that - a theory. In fact, all it is is just fan speculation. People heard that Plagueis could "create life" so they concocted this wild rumor.
First of all, only the force can create life, so technically all Plagueis would do is coax it to do so.
Even if Plagueis did do it, I don't see that as some sort of trump card that you can just play against everything Sidious did. Just like TKA said, Darth Sidious had the force powers necessary to kill three Jedi masters in seconds, control vast force storms, cloud the vision of the Jedi Council, and achieve immortality. If you couple that with his vast political prowess then Plagueis's supposed ability to impregnate random slave women with his mind is still not enough for me to change my mind and declare him the greatest Sith lord ever.
Plagueis would need way more than just that one accomplishment in order for him to be in the running, and right now, he doesn't even have that. Heck, even his ability to save others from death is still debatable.
Also, say he did cause Anakin Skywalker to come into being. Shouldn't Anakin Skywalker, not Darth Plagueis, get the credit for bringing the force into balance?
Also, correct me if I'm wrong, but doesn't the prophecy of the Chosen One predate the life of Darth Plagueis? That means that even if Plagueis caused the birth of Skywalker, he was just chosen by the force to carry out that task, etc.
Personally, I don't see why the force couldn't have just done it by itself.
If they actually do come out and say that Plagueis randomly impregnated Shmi Skywalker, I will . . . well . . . nothing. I lost my respect for the EU a long time ago.