I couldn't possibly narrow it down to just a single game for each generation. 5th it would be impossible to do it, there's too many amazing games. 6th gen it would be easier to narrow it down, but still no way to choose a single game. 7th gen, by far the easiest generation for me in terms of picking favorites, but I still couldn't pick just one.
Math problems are the ultimate trick question. No matter how straight forward and simple they seem, the answer is never what you think it is.