First, eliminate Britain, Napoleon, Oynk, and Si Xue.
These folks would gladly interrogate him—how could they possibly help him avoid being interrogated?
Next, eliminate Red Ghost and the Potato Sisters.
Red Ghost is too powerful; the Three-Inch Mirror wouldn't even survive reflecting it—it'd shatter immediately.
The Potato Sisters are too weak; even if their inner darkness is copied, it wouldn't be of much use.
So that leaves Doctor Nile, French Plane Tree, the novelist, Sha Jiang, the Fisherman, and Niu Ma.
Among these, Sha Jiang is the strongest, Doctor Nile and the novelist are the weakest, while Niu Ma has the best relationship with him.
If he were to approach Niu Ma, it would definitely refuse at first. But as long as Liu Zheng uses the money tactic, he could still easily get it to agree.
However, always targeting Niu Ma feels a bit boring, and Liu Zheng can already predict what its inner darkness would look like.