The comparison between Rimuru Tempest, the protagonist from the light novel "That Time I Got Reincarnated as a Slime," and The Darkest Knight, which refers to a version of Batman who has been transformed into a powerful being in the comic book universe (specifically from the "Dark Nights: Metal" storyline), often depends on the context of the encounter.
Rimuru is an exceptionally powerful character with abilities such as reality manipulation, regeneration, and a vast array of skills obtained from the various creatures he has absorbed. He is known for his intelligence, strategic planning, and versatility in combat.
The Darkest Knight, on the other hand, is a version of Batman who has undergone significant transformations and possesses powerful abilities, including cosmic powers and the utilization of various entities like the Dark Multiverse's powers. He is portrayed as incredibly strong and resourceful, with access to technology and tactics that make him a formidable opponent.
In a direct confrontation, Rimuru's overwhelming magical abilities and regeneration might give him an edge. However, the outcome could vary based on the circumstances, such as the environment, available resources, and specific rules governing their abilities at the time of the encounter.
Ultimately, it comes down to personal interpretation and preference within fan debates about who would emerge victorious in a hypothetical battle.