How to Sort Specific Options in Drop Down List

I mentioned this in a local group chat, but if you really want the "most commonly selected" options at the top then you can add a Numeric Summary lookup field to the relationship. Then set your report that powers this list to sort that field in descending order. More relationships, the higher up it will be on the list.

It really depends on what you want your final list to look like. Roy's suggestion is the common solution, just give yourself control over the sort order with an additional field.