Most of my work is in food & beverage, and Wonderware seems to be the biggest player there. I've seen a bit of Citect about too, but not anywhere as much as wonderware.
I've heard that Honeywell Experion is big in oil/gas/energy. Can't vouch for it personally, but I hear it from multiple places, and don't really hear much about anything else being used, so it's definitely on the list.