Nearest neighbour heuristic 2048

Im trying to make a AI solving 2048. I currently use 3 different heuristics: Clustring (Clusters big tiles in the upper right corner), Sum of Zeros(having as much empty space as possible) and -Sum( trying to get as "big" tiles as possible merging.

I'm trying it implement a Nearest Neighbour heuristic aswell. But im really not sure how to do create it. If anybody has any tips i would highly appreciate it.