Formal:
* Como está? (This is the most formal way, used with strangers or someone you want to show respect.)
* Como vai? (This is also formal, but slightly less formal than "Como está?")
Informal:
* Como vai você? (This is the most common way to ask someone how they are, used with friends and family.)
* Tudo bem? (This literally means "everything good?", and is a quick and casual way to say "how are you".)
* E aí? (This is a very informal and casual greeting, similar to "What's up?" in English.)
Other options:
* Como se sente? (This literally means "how do you feel?", and is a more personal way to ask how someone is.)
* Como tem passado? (This literally means "how have you been?", and is used to ask someone about their well-being over a longer period of time.)
The best way to choose the appropriate phrase is to consider the context and your relationship with the person you are talking to.