Wouldn't a more natural way of saying this be: 你最喜欢的动物是什么？
I seldom hear 最爱 being used in China
Is there another more idiomatic way of expressing 'favorite' that you hear?
There is also 喜爱.
This answer is now accepted.
why is the english translation so loose? 你最爱什么动物 literally says 'what animal do you love the most'. Because 最爱 here is a verb. It could be an adjective if used like 你最爱的动物是什么
Is there any reason why one couldn't say 哪只动物 instead of 什么动物?