It is very difficult to determine what would be the best selling quilt pattern. I remember quilting from the depression days when Mom and friends had a Quilt Guild. They worked on 9 patch, 4 patch both with sashes, Grandmother's flower garden, Pickle dish, Wedding ring (both double and single), sometimes applique, mostly tulips and roses and square on square for the wool quilts. There were no purchased patterns that I know of. They just traded patterns back and forth for the quilt the Guild was working on at the time.
If I had to choose what the best selling quilt pattern is, I would pick Trip around the World.