best ab machine/device

what is the best device for working out your ab muscles

I don't like ab machine/devices. Doing various of ab workouts will strengthen your abdominal. However, if you have fat you will not be able to see the ab muscles you have gained.

You don't need to use machines to work your abs, there are plenty of exercises that require no additional weight other than your body.
On top of sit-ups and crunches you have planks, side planks, and leg lifts just to name a few.

Books.
I used hard-cover encyclopedias as weights as I do 10 sets of 30 reps sit-ups.

This, plus a few of those ab machines have a a reputation for damaging ones back. Often one will have bad posture when using one due to being lazy and think that the way they sit doesn't matter.

The human body

I use a simple bench(slightly inclined) as I do situps while holding a dumbbell on my chest.
What's important is having the proper form.
Just remember that if you do it slowly, your building your muscles more effectively.
If you do your situps with speed, then your building more for endurance and for lean muscles

Good old-fashioned crunching (along with a handful of relevant variations) seems to still be doing the trick!

Elastic resistance bands or any kind of weights you can put atop your body while doing situps. What's important is having the proper form.