squats, deadlifts.. planks build ab stabilization. Remember, the abs number one task is to stabilize the body, not move. While doing squats, deads or planks you train the stabilization, which is what you need in daily life.
Now if you care mostly about looks, its another story, but situps still aint all that good. Weighted crunches perhapps. Do you rise your lowerback from the floor when doing a situp? like going ALL the way up? you shouldnt do that.