Alright now, relatively speaking normally when someone asks who the strongest is yada yada yada, they always ask about characters who either A.) Weren't around when so and so was, or B.) So and so was either too young or too old to be able to stop the other.
Now for the Sith, with how they say there can only be 2, a Master and Apprentice, that Rule was established by Darth Bane, called "The Rule of 2".
But now for the topic at hand, my "Top Ten" Sith, Both Cannon and Legacy (there is no bullshit of this filler shit because Disney decided to say f*ck you to an established history)
1.) Darth Sidious: This guy took on 4 Jedi Masters, at the same time and lived to tell the tale about it, albeit very injured at the end of it. As well as manipulated Anakin for years molding him to become a Sith Lord. Also he managed to turn one of Yoda's pupil's into a Sith Lord, Count Dooku, a.k.a. Darth Tyrannus.
2.) Darth Maul: Only reason I am placing him here is because he is the only man to have ever killed Liam Nesson on screen. Also this guy survived getting cut in half by Obi-Wan Kenobi, after having killed the Jedi Master, Qui-Gon Jinn. As well as trained his own brother to become a Sith.
3.) Darth Vader: This guy was the one of the Greatest Jedi during the clone wars, but sadly gave into the Dark Side and oddly enough became somewhat weaker for it. Anakin was quick, nimble and could easily fight. But as Vader he became slow and only relied on the Force for damn near everything, albeit very strong with the force but not as strong as he was.
4.) Darth Bane: This guy basically took charge of the Sith and established a rule that has lasted a very long time.
5.) Darth Tyrannus: A.k.a. Count Dooku, Leader of the Separatist Army. Trained a Cyborg Kaleesh in the art of the Lightsaber, Master to two Dathomirian's who were powerful with the force, even though he was betrayed by both. Was the Master of Qui-Gon Jinn, and trained several others in how to become a Dark Jedi.
6.) Darth Plagueis: Alright I don't know too much about him, haven't done enough research on him yet, just know what the movies have told us about him.
7.) Savage Opress: Older Brother to Darth Maul, physically one of the strongest Sith ever shown. Was turned into a Nghtbrother on his homeworld of Dathomir thanks to Asajj Ventress (both Asajj and Savae were trained by Tyrannus although Savage's time was quite brief.) Savage even managed to hold his own against Sidious before he muffed up and got killed for it.
8.) 9.) & 10.) Alright these three I will have to put as Darth Nihilus, Malak, and Revan, only because I really don't know too much about them.
Now as for Kylo Ren, sadly he is not a Sith yet. Yes, he does dress like one and has a red lightsaber but no he is not one.......yet