Most popular, I'm not sure. Most used? In the Deep South, I think Rose of Sharon or as we call it, bush hollyhock, is most used. When I was growing up in the 1950s and 60s, almost every home in the city and every home or farm in the suburbs and country had a hollyhock bush. They were usually white, blue or purple with 5 petals. You could plant them and forget them.

Hala,
In the NE, USA both Rose of SHaron (cold hardy) and Tropical hibiscus sell great.
People here bring in their tropicals for the winters and take them outside in the summer.
I have not seen Hardy hibiscus seeds or plants for sale too often in PA. I'm sure lots of people plant them inside over the winter to grow seedlings and then move them outside in the spring, I just don't think they are as popular as the other 2 varieties.
Hollyhock is not Rose of Sharon. Two different plants.