So like, spoilers warning. Duh.
Everyone is throwing around theories about who Supreme Leader Snoke is, if anyone aside from being Snoke, and a lot of them have to do which Sith Lord he is. There's some good ideas being thrown around, but the most intriguing theory that's out there is that Snoke is Darth Plagueis, AKA Palpatine's mentor.
Without going too far into who or what Darth Plagueis is, it is important to note that of the few things said about him in Episode III, he is called Plagueis the Wise. He was called wise because he had figured out how to cheat death.
Notice then one of the few things said about Supreme Leader Snoke in Episode VII. When Han Solo is Confronting Kylo Ren on the catwalk, just before, well, that one thing happens on the catwalk;
If you listen to what Kylo Ren says about Snoke when Han is confronting him about the Supreme Leader's influence, Kylo insists that he is wise. Not very wise, not pretty wise, and not a wise leader. Is wise. as in Snoke = wise. Remember now what was said about Plagueis? The wise.
I mean, think for a second the number of other adjectives could be used for a good mentor: he could have been smart, intelligent, insightful, sage, skilled, or knowledgeable. Or it could have been accompanied by any number of adjectives. So the fact that Kylo Ren calls Snoke wise, makes for the strongest bit of evidence that he is actually the old dark Sith Lord himself.
I mean, just because you can prolong life doesn't mean it has to look pretty, and Snoke looks like he's dealt with the business end of a lightsabre at least once.
So there you have it, the short sweet look and the strongest argument that Snoke is Plagueis. I hope this makes your day better.