My socks wear out first under the base of the big toe, then under the base of the little toe. After that, the rest of the ball of the foot, under the big toe, and along the edge of the toes. (That last one is trickiest to mend, along with the tip of the big toe, and will be the place that finally makes me bin a pair.)

Never at the heel. On some pairs I can be close to needing to do a second layer of darning on top of a previous mend under the ball of the foot, and the heel is still all good. Possibly a bit felted, but never worn through.