Sit ups are going to work your Hip Flexors primarily unless you strive to add waist flexion during the movement. Your Rectus Abdominus (Abs) will only work isometrically during Hip Flexion.
To work your Abs directly you need a full contraction which can only occur during waist flexion. Crunches/Reverse crunches are the best for that.
Every day i see people on the Captains Chair doing leg raises, thinking that they're doing an Ab workout but only working their Hip Flexors instead. The same goes for a lot of 'Ab' work i see, like Sit-Ups. If you're doing leg raises on a Captains Chair and your lower back isn't coming away from the pad, i.e your body isn't curling into a ball at the top of the movement, you're not working your abs.
If you do sit ups with a straight torso, bringing your back up flat, you're only working your Hip Flexors. Again, you should be bending at the waist, your shoulders should be coming towards your knees and you should be rounding your back. That is how to work your Abs.
Stick with Crunches, keep your lower back in contact with the floor, round your back as you slowly bring your shoulders toward your knees and hold the contraction before slowly lowering your shoulders back down.
Also, if you want your Abs to grow, the same principal of progressive overload applies. If you can do >20 reps with good form, you should start adding resistance by holding a weight behind your head during the exercise.
Personally i get the best results with Bicycle Crunches. YMMV