What is the comparative and superlative degree for silent?

The word "silent" is already in the positive degree. It doesn't have comparative or superlative forms because it describes a state rather than something that can be compared in terms of degree.

You can, however, use other words to express different levels of quietness:

Comparative:

* Quieter

* More hushed

* Less noisy

Superlative:

* Quietest

* Most hushed

* Least noisy