XSUPREMEXSKILLZ
The Immortal Emperor
The way I see it, Snoke's strength is contingent on the strength of Kylo and Rey. and oh boy, Kylo and Rey are strong as hell.
As Ellimist has said, Ren has sheer power/potential that Luke has only otherwise seen in Rey (who is Ren's equal in the light). Furthermore, Ren has a lifetime of training and practice, hence why he's one of the most powerful characters in galactic history:
Kylo Ren has a lifetime of training and practice, is one of the most powerful combatants in the history of Star Wars, and is the leader of the Knights of Ren.- StarWars.com
Snoke says at the end of TFA that he's going to "complete Kylo's training." What did this training entail, though? As we see in TLJ: resolve. Snoke knew that Ren hadn't fully committed himself to the dark side, and that the deed of killing Han Solo essentially split his spirit in half. Snoke felt that Kylo would become the next Vader once he'd fully given himself to the dark side.
That's it.
Kylo has all the skills, he's a Skywalker who's been training all his life. He has the raw power. He only lacks resolve. This is why Snoke essentially starts wanking the shit out of Kylo before he dies: he sensed that Kylo was becoming the true heir apparent to Lord Vader by no longer being conflicted and being, like Anakin against Dooku, as clear as a crystal bell mentally.
Heck, one could liken Kylo to Mustafar Vader in that sense. Anakin was overwhelmed by fear, and as Rey notes in TFA, Kylo had a crippling fear that he'd never be as strong as Darth Vader.
Aside from injury, that's also why I think Rey was able to defeat Kylo in TFA. Not only is Rey Kylo's equivalent on the light side (meaning that as Kylo grows stronger, as does Rey, at least according to Snoke,) but she has a degree of resolve that Ren certainly didn't possess. Snoke notes this multiple times, (still that fiery spit of hope! You have the spirit of a true Jedi!).
And yet, despite all of this, Kylo and Rey's current power levels, as Ellimist noted in the OP, aren't exactly close to that of Snoke.
Of course we see Snoke casually meld Kylo and Rey's minds from across the galaxy, to the point where they are connecting physically, (Ren notes that such an exertion of power would kill Rey, who is his own equal). We have Snoke overpowering Rey's grip on the lightsaber with the utmost ease, ragdolling Rey essentially without moving, and easily ripping all the knowledge he wanted from Rey's mind, (remember when Kylo was overpowered in a TP bout with Rey when she didn't even know she could use the force?). Keeping in mind that even after Kylo found his resolve, his applicable strength in the force was still on par with Rey's given their struggle to gain the lightsaber.
Ultimately, a lot of this is subjective, but the way I see it, Ren is extremely powerful and skilled, Rey equally so, and Snoke's power visibly dwarfed both of theirs.