* Cosa desidera? (formal) - This is the most polite and formal way to ask. It's appropriate for a waiter in a restaurant, a shop assistant, or someone you don't know well.
* Cosa vorrebbe? (formal/informal) - Similar to "cosa desidera," but slightly less formal. It can be used in most situations.
* Cosa ti piacerebbe? (informal) - This is the most informal option and should only be used with friends, family, or people you know well. "Ti" is the informal "you."
* Che cosa vuoi? (very informal) - This is a very informal and direct way to ask. It's suitable only for close friends and family.
The best choice depends on your relationship with the person you're asking.