What is the proper transformation into a superlative form for performance?

The appropriate transformation into a superlative form for "performance" is "That was the best performance." This choice effectively utilizes the superlative adjective "best," signifying that within the context given, this performance stands out as the highest quality in comparison to others. The superlative form is specifically designed to express the highest degree of a quality, which in this case indicates that no other performances exceeded this one in quality or excellence.

Other options do not convey a true superlative meaning. The second option, "That was a good performance," uses a positive adjective rather than a superlative. The third option, "That was one of the better performances," suggests that the performance ranks among the higher quality ones but does not assert it as the top performance. The fourth option, "That was an excellent performance," uses a strong positive adjective but still falls short of indicating that this was the very best among all performances. Thus, the first choice clearly stands out as the proper transformation into the superlative form.