Final answer:
The most useful stain for staining the leprosy bacillus is the Acid Fast stain.
Step-by-step explanation:
The most useful stain for staining the leprosy bacillus would be the Acid Fast stain. Acid-fast staining is a differential staining technique that is used to identify bacteria that have waxy mycolic acids in their cell walls, such as the leprosy bacillus. The acid-fast stain is capable of retaining the primary stain even after the application of a decolorizing agent, allowing the bacteria to be easily distinguished.