Are sit-ups the best exercise to prevent back pain?
No. For many people, situps CAUSE back pain because people don't do them properly. They lower themselves the final few inches to the ground by arching their back....Trouble.
The "crunch" is supposed to eliminate this tendency, since the back stays pretty much flat on the ground.
The best (according to most authorities) low-back exercise is the simple dumbell-swing between the legs while standing.
You keep your back straight, and use a light dumbbell.... You can use one or two hands. If you use one....Just toss the dumbbell from hand to hand on the upswing phase.