You might want to consider altering the sentence to a non-passive voice so as to read... Banana is my favorite fruit. or Bananas are my favorite fruit.

This looks fine to me...

Do you intend to stress the fruit or that it is your favourite? If the fruit, then say "Banana is my favourite fruit". If the favourite fruit part you want to point out/stress, whatever the fruit it is, then say " my favourite fruit is banana".

You have to say bananas. It seems strange, but: You can't say banana - that's uncountable! And 'My favorite fruits are..' needs more than one answer. It's just a weird thing about English, ha ha. :)