Death should be the most powerful imo. Even more than the primal devils.
How the other horsemen stack up to the primals, I am not sure. Makima was wounded heavily by the Darkness Devil. Tho at full power and proper setting it could be more intriguing.
Death is gonna be the ultimate terror that Fami spoke of, I believe, and the strongest, cuz imo it makes the most sense. I'd also wager that Death is gonna be Fami, Yoru and Makima's mother perhaps, cuz she would be more powerful and represent the first and biggest fear, and is gonna be like the mother of devils.
We were told iirc that devils appear on Earth when they die in Hell and are reincarnated. That's why the primals are so powerful, cuz they never experienced death
That's why I think Death might be killing them now so they appear on Earth.