If you wanted to find out more about a particular bike but you had no idea how it looks and what it is and you could ask only one question, about only one of that bike's component, what would it be?
"What frame is it build on?" - you may ask. But I think that my question would be about tires. Finding out what tires are installed on a bike tells me pretty much everything I need to know: the size of wheels, likely the type of frame, the conditions that bike would be used for, etc.
After I thought about it a bit more, I think that a bicycle tire is that single component that tells the whole story.