An editorial answer that addresses the packing concern without inventing a ranked survey of forgotten belongings.
1 answer
There is no verified universal ranking here that establishes one item as the most forgotten. The more useful question is which omission would be hardest for you to recover from during this particular trip.
Make a short high-consequence list: the documents you need, your regular prescribed medication, essential glasses or contact-lens supplies, and access to payment. Then make a second list for annoying but usually replaceable omissions, such as a charging cable or toothbrush. These are examples of priorities, not claims about what travelers forget most often.
Items used on departure morning deserve their own final check because they may never enter the bag during the main packing session. Put a note beside your keys naming anything still charging or still in use. Physically touch each item before crossing it off rather than relying on the memory of seeing it yesterday.
A destination check helps as well. Think through arrival, the first evening and the following morning. If an essential item is missing, that short rehearsal often reveals it. Keep the resulting checklist for your next trip and update it from your own actual omissions.