Good question. I'm trying to figure out the same thing. It's hard to find any exercises that you can load enough weight to make it not an endurance thing.
Disregarding back issues, I always found a weighted crunch machine the best exercise to build strictly abdominal muscle. For overall core strength and safety, I believe planks to be the best. Theses exercises will not lead to a tapered look though. That takes diet.
Yeah some of us are cursed/blessed with wide hips, thick waist, big core area in general... great for strong lifts but not so great for the classic v-shape.