In the provided ranking of splash sizes, the ranking is as follows:
- W: 2
- X: 4
- Y: 1
- Z: 3
The largest splash size is attributed to object X, which has a ranking of 4. Given that the objects are of the same size and dropped from the same height, a larger splash often correlates with greater mass. Therefore, it can be inferred that object X most likely has the greatest mass.
So, the response is: object X.