My understanding is that the best thing for abs are planks because the primary function of abs is stabilization so the curling movement like situps/crunches aren't nearly as effective as stabilization exercises like planks and compound lifts. Just my understanding though. When I used to do situps my favorites were the exercise ball situps because they offered a greater range of motion and I couldn't do roman chair situps.
Feeling a burn does not make an exercise effective. I am told that those suffering from gonorrhea often feel a burn. So what?
Russian twists (I prefer to do them on a decline board so I can use more weight), Saxon side bends, decline medicine ball sit-ups/throws, barbell roll-outs, standing barbell rotations, ab pull-downs.