What are the comparative and superlative of word often?

The word "often" is an adverb, not an adjective. Adverbs don't have comparative and superlative forms in the same way that adjectives do.

To express different levels of frequency, we use other words or phrases:

More frequent:

* More often

* Frequently

* Regularly

* Usually

* Most of the time

Most frequent:

* Always

* Constantly

* Continuously

* Perpetually

So, instead of saying "more often" or "most often," we use these alternative expressions to convey the desired meaning.