it really doesnt matter...stick with the basics...heavy weighted decline cruches, hanging leg raises (especially good for the lower abs), cable crunches, EZ bar roll outs are some good ones. Try any of them and tell me a certain area of your abs arent being worked.
your lower abs, your 8 pack, whatever, isnt even going to be visible unless youre damn lean...single digit bodyfat is a requirement, and you might not even see them at 8-9. so yes...diet and genetics are gonna be the main aspect here...you can do crunches all day and develop the most well built abs on the planet, but you wont see them if youre 20% bf.