In terms of passenger travel through, but not into or out of, its airspace, what would be the number 1 "flyover" state in the USA?

West Virginia.

https://www.usatoday.com/story/travel/flights/2017/11/15/flyover-states/860668001/

According to the link above, in raw numbers they calculate Virginia, but if you consider flyovers as a proportion of aircraft entering the airspace, it is West Virginia.

However, their analysis ignores Delaware - with no instate airports, there are no landings to divide by in producing their ratio, so the table has no entry at all. Given its location on the busy east coast airway, it may well be competitive!