Based on the provided rankings of splash size, the object with the highest splash size ranking (4) is likely to have the greatest mass. From the given information:
- W: 2 (splash size)
- X: 4 (splash size)
- Y: 1 (splash size)
- Z: 3 (splash size)
Since object X has the highest ranking (4), it most likely has the greatest mass.
So, the answer is: object X.