@Hutchy: I don't think that list's in order
@Ozz: In legends, the big names are usually Palpatine, Vitiate, Krayt, Bane, Nihilus, Kreia, Exar Kun, Naga Sadow, and Freedon Nadd
By association, we get names like Dooku, Vader, Revan, Malak, Plagueis, Vaylin, etc.
In Canon, we haven't heard of too many sith lords yet, but Sidious still holds the title of "most powerful."
Vader follows immediately after.
Besides them, I can think of Lord Mommin (who designed Fortress Vader in ancient times).
There are other sith mentioned, but at the time they're featless and it's likely they were just average.
Originally posted by hutchy1345
Vaylin is that high?
I am not into ranking the strongest but TOP 3 are easy to decide. Each Force-user I have mentioned, have incredible demonstrations of power in different ways. This is why I make an unordered list because it remains open to interpretation. I forgot to mention Revan - he is definitely among the strongest.