What are the comparative and superlative forms of word fantastic?

"Fantastic" is already a superlative adjective, meaning it's already at the highest level of something. There is no comparative or superlative form of "fantastic" in standard English usage.

However, if you want to express an even higher degree of something fantastic, you can use intensifiers like:

* Absolutely fantastic

* Incredibly fantastic

* Remarkably fantastic

* Unbelievably fantastic

You can also use synonyms like:

* Amazing

* Splendid

* Exceptional

* Outstanding

Remember, using synonyms or intensifiers is often more effective than trying to force a comparative or superlative form on a word that already implies the highest level.