I'm afraid I would disagree with you.
"My favourite animal is a dog."
You must use singular "is" because "animal" is the subject, and "animal" is singular. The object must also be singular to agree with the subject of "to be". The only exception is for objects which can commonly be thought of together as a singular collection. This is true for "strawberries", but not "dogs", because we think of eating "strawberries" as a whole rather than the individual strawberries.
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