I hate abs, I feel them flex a lot when I'm struggling with that last squat and I sort of consider that enough.
This girl I work out with sometimes, you can't see them in even the best lighting, but that girl has some ABS. She does an abs circuit 3x a week:
Crunches (25 day 1 +25 each day)
Planks (60 seconds +10 seconds each day)
Sit-ups (25 day 1 +25 each day)
Russian Twists (30 day 1 +10 each day)
Crunches (25 day 1 +25 each day)
Planks (60 seconds +10 seconds each day)
Leg Raises (25-20-15)
I'm pretty sure I pulled an oblique the last time I did it with her, and I've yet to fully complete it.
This Summer I plan on doing abs and then I'll definitely be attempting this, I suggest you try it if you're looking for something because it hurts.