Formal:
* Bien, gracias. ¿Y usted? (Good, thank you. And you?) - This is the most formal and polite way to respond.
* Estoy bien, gracias. (I'm good, thank you.) - This is also a polite response.
Informal:
* Bien, ¿y tú? (Good, and you?) - This is a more casual way to respond.
* Estoy bien, ¿y tú? (I'm good, and you?) - This is another casual response.
* Más o menos. (So-so.) - This is a way to say you're not feeling great, but not terrible.
* Regular. (Okay.) - This is a simple way to say you're doing alright.
Other options:
* ¡Genial! (Great!)
* No muy bien. (Not very well.)
* Mejor que ayer. (Better than yesterday.)
* Fatal. (Terrible.) - Use this one with caution, as it's quite strong.
You can also add a little more detail to your response:
* Estoy un poco cansado/a. (I'm a little tired.)
* Estoy un poco estresado/a. (I'm a little stressed.)
* Estoy muy contento/a. (I'm very happy.)
Remember to adjust your answer based on your actual feelings and the context of the conversation.