imo i think ab machines targeted at upper abs is a good way to go for strength because you can stack alot of weight on them. there is also a great ab/chest workout you can ad into your chest training day. Im a big fan of curl bar/db pullovers across a bench. it gives a great stretch across your whole abdomen while working your chest.
if your interested heres my current ab routine once a week:
machine crunch: 90lb 100reps 30 second rest between sets
parallel bar leg lifts: 100 reps
heavy machine crunch: 170lbs dont know how many sets
decline bench crunch: 200 crunches 30second rest