Ab Glide Master XL or any of the clones that have the resistance
gears housed on the inside. (NEVER use on carpet, too).

Sorest I've ever been from anything is the first week of that thingy.

2. mix it up. Some pure sit-ups, some crunches, some side lateral
work like right elbow to left knee. (anything that makes an ab-wall "X")

3. do hanging ab exercises (pull-up bar). Bring both knees up to
abs. bring 1 straight leg up at a time and scissors the air.

All 3 combines for a very fit and totally hit abdominal wall.
Much more so than just killing any two of the given three.

HTH's
b.street