Best abdominal exercises

Sit-ups and ab machines at gyms and fitness centers work great, but the best abdominal builder I've found is one my daughter learned back in her body-building days. What's more...it's the easiest to do, at least for me. And as with sit-ups, there is no special equipment required.
You do need a floor and a couch or easy chair that goes all the way to the floor. If you lack those, use a wall. If your floor isn't carpeted, you might want to spread an exercise mat or even a blanket for more comfort.
Lay flat on your back with your feet propped up at a cofortable length on the bottom part of the couch, chair or wall. Move back or forward just enough so your calves are straight from the knees. Put your hands behind your head.
Now, crunch your lower body up slowly until your elbows touch your knees. If you can't quite make this as first, go as far as you can. Hold for a long count of ten. Again if you can't make it, stay as long as you can.
Lower your back to the floor for a relatively long count of three to five.
Repeat the exercise 10 times. Again lower your body, rest for another three of five count.
Now do three of the same type crunches but very quickly up and down. Lower your body and take a rest. Not a go to sleep rest, but enough time to really catch your breath.
The goal is to work up to a series of three sets. Unless you are in good shape at the beginning, you might have to work up to this goal slowly, too. Work up to do one set completely, in that case, then start on the second and then on the third.
If you are thinking body building or something to build a six-pack, do this exercise only every other day or even three days a week. By skipping a day or two (and doing exercises for other parts of the body if body building is a goal), your muscles (which always tear some with any exercise) have time to heal out in bulk at the back where you want them while also firming the abdominal muscles. If you don't want to "bulk" up, do them every day. The muscles in your back will strenghen and your tummy definitely will flatten.
As with any exercise, don't do this without a doctor's permission. And, at least in the beginning, don't push yourself to the actual pain level. You'll be a bit stiff when you start, but total pain will just prevent you from continuing this or any exercise. So will total exhaustion unless you want to reach the "burn" level of exercise where the high is valued-and the body is in good enough condition to recover more rapidly.