And as inefficient and expensive a wafer harvest shape as possible.
I can think of worse ones, actually, but they'd never be of interest to any camera user. Imagine five pointed stars, etc. Of any shape that might be of realistic interest for cameras, the round one probably does indeed take the booby prize.
Square would be efficient at the wafer harvest level, but inefficient in the camera, with substantial image outside the sensor (assuming the sensor is inscribed in the image circle)--though not as bad as a super wide rectangle, and maybe not even as bad as what we use now--how often would people crop to make a rectangle out of the square? If so they're ending up with a rectangle MUCH smaller than the image circle. A hexagonal shape (assuming you are using a laser to cut the wafer) would be a very good compromise, no wasted wafer space, and pretty close to round. The next question would be whether we want to lay the pixels
out in a hexagonal pattern instead of a rectilinear one like now.
But--back to reality--none of this will be happening.