For my money, ISO Decline Abs are THE best way to build abdominal strength. I'm not sure they are the best for hypertrophy, but before I implemented them, I felt very weak in the core doing deadlifts. Now I feel a lot stronger in the midsection and my deadlift has gone up big time.
Simply hop in the old decline situp board with a weight on your chest. Decline to the hardest position and hold for a set amount of time. You're not going to really "feel the burn" with this one as much, but I never really cared for that anyway.