1. What is your favourite season?
2. Which is your favourite season?
3. What is your favourite festival?
4. Which is your favourite festival?
Are the above sentences all correct? Which of them are more natural?
When there is a fairly short list of possible answers, "Which" works best for me, so "Which is your favourite season?" With festivals, it would depend on whether or not you know that the person answering the question goes to a small number of select festivals every year and you want them to choose a favourite from that list. If so, then "Which" again.
When there is a huge number of possible answers, I would probably go with "What" - What is your favourite film?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.