Few characters in Star Wars have had as much impact without ever being seen on screen than Darth Plagueis. Palpatine mentions him to Anakin in Revenge Of The Sith, saying:
"Darth Plagueis was a Dark Lord of the Sith so powerful and so wise, he could use the Force to influence the midichlorians to create life. He had such a knowledge of the Dark Side, he could even keep the ones he cared about from dying."
So, pretty damn powerful then. However, while he may sound a total badass, we never got to see him, as his former pupil Darth Sidious killed him (which in itself shows how powerful Palpatine was). But, wait, Snoke IS Plagueis, right? It's by far the most popular theory regarding the Supreme Leader, whose past is still a mystery, and it does make a degree of sense, giving another connecting thread from before the first film (and the creation of the Skywalker lineage) right through to this one. We may or may not get confirmation on that in Episode VIII, but one clue from The Force Awakens comes from the music: Snoke's theme in the movie is incredible similar to the music from Episode III, when Palpatine is describing his former mentor.