How do you say really good in spanish?

It depends what you're trying to say.

If you want to say 'The food is really good', you would say 'La comida está muy buena' or even 'La comida está buenísima' (really, really good).

If you want to say 'The weather is really good', you would say 'Hace muy buen tiempo.'

If you want to respond to someone's question 'How are you?' with 'Really good, thanks' (or 'Really well, thanks'), you would say 'Muy bien, gracias.'

It just depends.