What is your favorite subject?
cual es tu preferida, literally, means (as close as possible: which girl do you prefer? Cual prefieres (sp) is closer to: which do you prefer.
Tu materia preferida es matemáticas.
What's your least favorite class
Cual es tu telefono = What is your telephone number
cual es tu apellido means what is your surname so you would with respond with your last name.
cual es tu means 'what is your....' (informal)cual es su means 'what is your....' (formal)
Cual es tu nacionalidad means "what is your nationality". So you would reply with your nationality ("Mi nacionalidad es....)
Which is your sport.
Cual es el poblacion de tu pais means What is the population of your country
its writen as: cual es tu numero. it means what is your number
This means "What is your speciality?"
It means, " is which your phone "