Darth Plagueis Died By The Hands Of Palpatine
Let's start with common knowledge: In Episode III, Revenge of the Sith, Sidious is obviously old, and if he didn't kill Darth Plagueis when he was only an apprentice, wouldn't Plagueis have wanted Revenge and attempted to kill Sidious, not wait around for half a century and just wait until he dies to take over. From all that we know, an all powerful Sith Lord like Plagueis would not do such a thing. There is even evidence from Snoke himself, Andy Serkis:
"It is very much a newly introduced character," Andy Serkis said in an interview discussing Snoke.
Another reason that Snoke is not Plagueis, is that J.J. Abrahams was even considering making Snoke a girl. Do you honestly believe that Darth Plagueis transformed into a girl in 50 years? I don't.
Snoke as a female