The most important thing here is to answer relatively quickly, and back it up. One of the fallouts of this question is age. Answering "Robotron!" to a 20-something interviewer might lead to a feeling of disconnect. But sometimes that can be good. It means you have to really explain why it's the best game of all time. Can you verbally and accurately describe a game to another person who has never played it? You'll rack up some communication points if you can.

What you shouldn't say is whatever the latest hot game is, or blatantly pick one that the company made (unless it's true and your enthusiasm is bubbling over). Be honest. Don't be too eccentric and niche, and be ready to defend your decision.