Word of God (aka Lucas) says that before Anakin and Luke, Yoda was the strongest Jedi and Palpatine the strongest Sith. Palpatine actually has the honor of being the strongest Sith ever.
Lucas also says that Anakin had the potential to become the strongest Force user ever, but much of that was lost due to losing his limbs and whatnot. Luke, however, had the same potential, and did reach it, and Lucas has confirmed that Luke is the strongest Force user ever in the Star Wars universe, and that no one will ever surpass him.
And ignoring the Expanded Universe is lol worthy considering most of it is of higher quality than the prequels.
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Gamertag-Murphiroth
Pokemon Black-1978-9490-1161