From the collective consciousness I pulled the 2004 Toyota Corolla weighing in at 2,502 to 2,590 lbs. Because certainly there's no other way I would guess The the most average car so on-the-nose. And I have seen the videos about the Honda Insight being good for gas mileage (even back in 1999, it's a hybrid).
I guess actual price, availability, perception, mantenence etc. molds it for most people though.