Generally, the answer I give in response should have two words tacked onto the front of it: "Right now..." Because certainly, I have a favorite food in that moment. It might just be whatever I happened to eat last. It might be whatever I would like to eat next. It might be whatever I last saw pictures of while stalking the food blogs.
And certainly I have a favorite song at any given moment. What song did I hear last? What mood am I in? Which artist am I currently obsessed with (and who is my Favorite Band of the moment)?
I've never had a Favorite Book Of All Time. I have a top five list Of All Time, but it's impossible for me to choose between Rant and The Minotaur Takes A Cigarette Break, and I can't really compare either of those to Paradise Lost without making a mood-driven and arbitrary decision. And I forget all the time how much I love Catch-22, and then I re-read it and it fucks the entire list up. My favorite vegetable is broccoli, unless you just plunked a plate of brussel sprouts down on the table, in which case, back off, Buckwheat, they're mine.
I was once told - and I have no real evidence to back this up, but I was told it and it seems sensible - that if you ask someone from, say, Great Britain what their favorite color is, their reply will be, "Color of what?" You can't have the same favorite color for everything. Your favorite wall treatment is probably not the same as your favorite eye color in a sexual partner is probably not the same color you feel you look your best in.
If it is, I guarantee you'll say your favorite color is purple, and you're gonna be looking for someone with purple eyes your whole life, buddy. And when you find her, she won't be interested in you, and you'll find out after an ego-bruising amount of wooing that they were contacts.
Not that anything like that has ever happened to me.