Formal:
* Quel est votre nom? (Most formal, used for someone you don't know well or in a professional setting)
* Pourriez-vous me dire votre nom? (Slightly more polite than "Quel est votre nom?")
* Comment vous appelez-vous? (More common than "Quel est votre nom?", but still quite formal)
Informal:
* Comment tu t'appelles? (Used with friends and family)
* C'est quoi ton nom? (Very informal, like "What's your name?")
You can also use these phrases:
* Je ne connais pas votre nom. (I don't know your name.)
* Pouvez-vous me rappeler votre nom? (Could you remind me of your name?)
* Quel est le nom de famille? (What is your last name?)
The best choice depends on the situation and your relationship with the person you are talking to.