Erm... tough question! If I did have to bet on a country in the world, I suppose it would be the UK because I live here and it seems stable and reasonably prosperous generally. It just seems like 'thing to do' - to track the market of whatever country you live in as a default. For some reason I perceive tracking the world as riskier - maybe because I feel like I know what the UK economy is doing at any given time but I don't know what's going on in the rest of the world. Also, a vague and probably unjustified sense that if the COL goes up in the UK our stock market probably will too, but that won't correlate with the rest of the world.
So in summary: Not sure!