The one that was on every school computer that could still reach the internet.
Pong in three dimensions, played down a tunnel, with one addition that makes it: the ball takes spin from how your paddle is moving when you hit it. Strike it flat and it comes straight back. Strike it while sliding and it bends away from where your opponent expects, and keeps bending.
Which means the skill is not really reflexes. It is reading the curve early enough to be somewhere useful, and putting enough spin on your own return that the same problem becomes theirs.
