What dark type move deals the most damage?

Question

Try to explain the move as much as possible.

Answer 1

Z-Move
Malicious Moonsault
180 Base Power
∞ Accuracy
Physical
1 PP (Max 1)
Introduced: Gen VII

Malicious Moonsault is a Dark type Z-move exclusive to Incineroar. It is used with Incineroar's signature move Darkest Lariat.

If the target has used Minimize, Malicious Moonsault ignores accuracy and deals double damage.

Regular Move
Hyperspace Fury
100 Base Power
∞ Accuracy
Physical
5 PP (Max 8)
Introduced: Gen VI

Hyperspace Fury deals damage but lowers the user's Defense by one stage after attacking. The move always hits, even if the target used a Protect-like move.

If we're not counting signature moves (Malicious Moonsault and Hyperspace Fury belong to Incineroar and Hoopa-Unbound respectively), then the highest powered Dark move is still Foul Play.

Answer 2

Foul Play.
95 Base Power
100 Accuracy
Physical
PP 15 (Max. 24)
Introduced: Generation V
More on this link.

Foul Play uses the opponent's Attack stat instead of the user's in damage calculation. In other words, the move acts as if the target is attacking themselves, and thus is more powerful when the opponent has higher Attack.