Yes, I don't think it is that easy either. There isn't any property of a question that would make it obvious. The question number is approaching 4 million right now. I assume that the question count is fairly heavily cached, far too expensive to calculate for every page refresh. So the storm of page refreshes when it gets close isn't going to show the winner. And it is very dynamic with questions getting deleted all the time, #1M can be (will be) #999,999 a second later.
Knowing the SO team, I bet they've programmed some kind of custom trigger that flashes a Unicorn page up for at least 5 minutes so the number is stable for at least that long. Looking forward to it, hope it's a good question. And have the honor to get the answer mark for it :)