You would obviously know better than I, but isn't that the same as the actual answer?
Is that because the word order here matters, or is this an older message exchange, the answer was changed, and I'm just really, really late to the party, lol?
What is your favourite weather? ("what" is the subject) -> Яка ваша улюблена погода?
What weather is your favourite? ("what weather" is the subject) -> Яка погода ваша улюблена?
In English, 'Which is your favourite weather?' is better. 'Which' should be used, in general, when referring to items from definite sets.
It's accepted too. It's not "better", just has a different connotation/feeling.
Which is for limited sets. Weather is uncountable with infinite variations. It's only better if someone provides a limited set. E.g. "Warm, dry and sunny or dreich, which is your favourite weather?"