Formal:
* Estou bem. (Literally: I am well.) This is the most common and neutral way to say "I'm good."
* Estou ótimo/ótima. (Literally: I am great.) This is a more positive way to say "I'm good."
Informal:
* Estou bem. (Same as the formal version.)
* Estou legal. (Literally: I am cool.) This is a more casual way to say "I'm good."
* Estou tranquilo/tranquila. (Literally: I am calm.) This is a more relaxed way to say "I'm good."
* Tudo bem. (Literally: Everything is okay.) This is a very informal way to say "I'm good."
Other options:
* Estou bem, obrigado/obrigada. (Literally: I am well, thank you.) This is a more polite way to say "I'm good."
* Estou de bom humor. (Literally: I am in a good mood.) This is a more expressive way to say "I'm good."
It's important to choose the appropriate phrase depending on the situation and your relationship with the person you are talking to.