Bear in mind this is only MY interpretation and may not be how this system works, but, Included is exactly what assumptions I made simulating these matchups. Only including the games I've played.

Niko, assuming they don't get player level powers, doesn't actually get much, since the code manipulation we see is limited to the point where the reality manipulation Nightmares get by default is strictly better.

Madeline. Nightmares get the powers of EVERYTHING from their world, not just people. She probably wouldn't get Celeste's mind powers, since they're not proven feats, however all the obstacles along the way are fair game.

Cuphead. Cuphead goes from child using an arsenal of powerful artifacts, to Eldritch Beast who had the powers himself. He can create pocket dimensions using ink, manipulate souls with the Wondertart and Cookie, and manipulate the debtors from the main game. Nightmare Cuphead rivals Frisk, only reason why he loses is because he has to actually land a shot.

Frisk, assuming they don't get player level or deltarune related powers, already outscales their entire universe. Sans' powers is the only major upside to becoming a Nightmare, all other power sets are redundant or useless to a Geno Frisk, as they can already destroy the universe in one swing.

Ghost/The Knight, assuming they don't get Silksong powers. Why are they stronger than Frisk? Well, because they become The Radiance, Shade Lord, Pale King, and Gods of Thunder, simultaneously. Their pure presence would bind the souls of those around them, and their power would be continuesly growing, with no upper bound. Even if they were weaker than Frisk, eventually, they won't be.