If the silencing goal is to stop the propagation of sound, then the answer is vacuum. Sound waves propagate through mechanical conduction, and for us humans, it is usually vibrating air molecules. Since a vacuum has nothing to vibrate, there is no sound propagation. However, there are two points to note.
In practice, a vacuum must be contained in a solid surface, such as a jar, wall, etc. That solid surface can still transmit sound. Similarly, a vacuum does not conduct heat (although heat will still be transferred through radiation, which is how sunlight heats us through the vacuum of space). This is the principle behind vacuum flasks. The vacuum layer between the internal contents and the outside prevents heat from entering and exiting, thereby keeping the contents hot or cold. However, after a few hours, the contents will still approach the ambient temperature, that is, room temperature. This is because the vacuum is contained inside glass, metal or plastic, and these surfaces still conduct heat.
The beginning of the problem is the requirement to achieve a "sound absorption" effect. Technically speaking, vacuum is not absorption. It does not absorb or suppress sound waves. On the contrary, vacuum stops sound waves, period. On the other hand, rock wool boards with rubber boards absorb and dissipate vibrations. Therefore, if you want to ask about a better absorbing object, almost anything is better than vacuum, because vacuum does not absorb. But if you are asking about the propagation of sound, nothing can stop its conduction like a vacuum.
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