"Bless you."
This is a traditional expression that wishes the person good health and well-being.
You can also say:
* "Gesundheit." This is the German word for "health" and is also commonly used in English.
* "God bless you." This is a slightly more formal version of "Bless you."
* "Hope you feel better." This is a more contemporary and empathetic response.
It's always best to be polite and considerate when someone sneezes, as it can be a sign of illness.