Formal:
* Eu estou fantástico(a). (This is the most literal translation and is suitable for formal settings.)
Informal:
* Estou ótimo(a)! (This is a more casual and common way to say it, especially among friends.)
* Estou super bem! (This is a very enthusiastic way to say it, implying you are feeling extremely well.)
* Estou numa vibe ótima! (This means "I'm in a great mood," and is a more modern way to express your feelings.)
Other options:
* Estou fabuloso(a)! (This is a more dramatic way to say "fantastic" and can be used when you are feeling particularly good.)
* Estou incrível! (This translates to "I am incredible!" and is a very positive and enthusiastic expression.)
The best choice for you will depend on the situation and your relationship with the person you are speaking to.