It took me a while to understand the differences. But here is what I have learned:
After "ser", the word "Qué" is asking for a definition. It is for open-ended questions.
"Cuál" is asking for a choice from a limited number of options (it is not open-ended).
To ask for a choice within a group (such as a group of sports), "qué" must be followed by a noun. ("Qué deporte prefieres"? Which sport do you prefer?)
Which is your favorite sport? "¿Cuál es tu deporte favorito?"
See these references:
Based on this RAE dictionary, it appears to me that the meaning of "preferito" is better translated as "preferred."
Del lat. praeferre 'llevar delante', 'anteponer', 'preferir'.
what's the sport you prefer 27/12/19 Though DL did not like this translation, and after reading the comments about the preferred translation of preferido as favorite, I think this might still be a good translation. Native speaker, jump in here and help us out of our ignorance.
When I listen to the sentence in slow mode, I can hear that it is "cual es" but at normal speed, it sounds like "cuales" which would still make sense. "What is your favorite sport" and "Which is your favorite sport" both mean the same thing. The word "cuales" was also recently introduced, so that's the word I used and, of course, got it wrong.