Maul. That clone of Maul almost beat Vader that was post TFU II Vader, who at the time grew strong enough to beat down Starkiller's clone who was stronger than the original, only losing because of plot.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
It occurs in infinities iirc, which wasn't canon even in Legends
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
No, it was a C-canon comic, but it was a clone of Maul, not the original one, so we have no idea how powerful he is.
In either case, Starkiller probably can't ragdoll Maul outright, but he just needs to wait for him to slip his guard slightly to bust out the telekinesis and FL, and he's good enough with a lightsaber to do so. (think: Dooku vs. Obi Wan)