Ab Gym Equipment?
All of the above?
Ab exercises, and core work in general. Most of it is done standing, not all of it is seated. I swear by bicycles myself.
Only equipment I've ever used is resistance bands. Wrap around the tops of your feet and have the ends of the bands coming out in between, and do what amounts to rowing motions with your legs in the air with your knees at a 90 degree ankle. Basically you move your feet forward while leaning back (but keeping good posture!), and the bands add the resistance. You'll feel those too.