Well, there was this bright light and then someone smacked me.
Frankly, I've always though this question to be one of the more useless things you can ask someone. It's almost impossible to know what our earliest memory is since it's often shaped by what others have told us. It's like how repressed memories are often found to be full of shit. When our families tell us stories about how we were as a kid, we have this tendency to internalize those events and actually invent memories of them. I think my earliest memory involves playing in the snow but I've heard my parents describe that day and there are photos that I've seen. At this point, it's impossible to know whether I actually remember anything or if it's all been cobbled together after the fact.