"Beth yw dy hoff fwyd?"

Translation: What is your favourite food?

[ajHenrot]

Would "What is your preferred food" also be correct?

[rmcode]

No, we would use the adjective 'gwell', so that would be 'beth yw dy well fwyd' or more commonly

'Pa fwyd sy'n well gyda ti' = 'Which food do you prefer'