My favourite food is apple - if you really mean that apples are your favourite FOOD above all others. If you just mean that it is your favourite fruit, then you would substitute 'fruit' for 'food'.

Ah...I see. Thank you for your kind help, Su.Ki.! I'm very grateful!

I'm afraid I have to disagree with Hilary. For me, none of those are right. I don't think that 'My favourite fruit is apple' can be correct. 'Apple' is a singular countable noun, so it needs an article. Merlyn is right that 'apples' would be better - but it doesn't depend on the country. The basic grammar of English is the same everywhere. 'My favourite fruit is apples' is preferable, but it doesn't sound good because of the mismatch between the singular verb and the plural subject. So what's the correct form? Quite simply - "Apples are my favourite fruit."