He never was, in Legends or in Canon.
Now in Legends, he did have the designation of being the most powerful Sith Lord up till his point in time (which covers a lot of ground, chronologically speaking; almost the entirety of the Star Wars legendarium), but he's never been shown, said, or frankly even suggested (to my knowledge, at least) as being the strongest Sith Lord who ever lived.
That being said, since there are barely any Sith Lords in Disney's canon (and even when some are named, barely anything power-wise is actually displayed or known about them) he may hold that position all the same, more out of default than anything else.
We also have a third issue, tho, and that's Disney's bizarre power ranking and scaling (iirc, Disney placed suited Darth Vader as being more powerful than Palpatine). It's all over the place right now, so there's really no telling
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