The last time I was asked that in an interview I said:
"Skittles. No, wait, chocolate covered blueberries. Well, snacks in general really."
I'm still working there 10 years later.
It's a bit of a silly question to ask I think, mostly people are going to give you some answer they've heard is a 'good thing to say at interviews'. It's really only helpful if people are actually honest when they answer, and many people are too afraid to be in that situation. Is there anything worse than coming up with an answer to that and having the interviewer frown, say "hmmm" and write something tiny on the paper behind their hand? Eeek!
Now I'm doing interviews, often the persons resume shows they have the skills and experience to do the job and I'm really looking for more of an insight into their personality, so instead of asking that question I say things like: "so, what would your greatest baking skill be? We like to snack here." Then when they answer I write in big letters on my paper "CAN MAKE SNACKS FOR OFFICE" with a tick next to it. They almost always laugh and it breaks the ice for the rest of the interview. I find they're more relaxed at that point and able to talk a little more freely, that then usually gives me a better insight into them as a person than the usual stilted job interview answers.