I bought one of those ab wheels that have been around forever. The first time I used it my abs were incredibly sore for a long time. It sold me on the movement. The abs have a really short range of motion. Sometimes I think the visual abs function as a stabilizer for the hip flexors and nothing more.
So planks, ab wheel, pulley crunches, and even push ups are really good movements for abs. Situps and leg raises train the abs but try the cheap ab wheel. Do it from the knees and as you advance from the ass up push up position to extension if you're good.