.38 Super used to be the most popular around here for Open Division, but cost and availability of cases has made that caliber so 1990's
Open is literally dominated by 9mm major now that all the open division gun manufacturers offer the caliber. It's a good thing since separating .38 Super from 9mm brass at the range was a nightmare for all involved...
My local racegun builder told me he hasn't built an Open gun in anything but 9mm in the last two or three years. A lot of his work consists of rebarreling Supers to 9.