There is no best answer. There are however facts:
1. SIt-ups are not the best exercise for your core and are definitely not the only exercise. They should form part of a good selection of core exercises working the muscles from different angles and not only concentrating on the stomach.
2. You can't spot reduce. This means if you want to lose weight from your stomach, just doing any stomach exercises won't do it. Your body stores fat in different places, once it starts using these fat reserves you don't get to choose where it should disappear from first.
3. To get a six-pack requires a very low body-fat percentage. For the vast majority of people exercise alone is not enough - which leads on to the old adage....
4. Abs are made in the kitchen. You can train as often and as hard (or as efficiently) as you like but unless you have a very good diet you are unlikely to notice your abs when looking for them.
5. You can do as many core exercises as you like - this can improve all the muscle tone, but if you have a layer of fat around your stomach no one will see that muscle tone. So no matter how many sit-ups (core exercises) you do you still won't see your six-pack.
If you would like to have great abs you need to be prepared to work at it.
If you need help get in touch for a free consultation.