So what is the best exercise for great abs?
Dragon flag https://greatist.com/fitness/dragon-flag
Human flag https://www.onnit.com/academy/so-you-want-to-do-a-human-flag...
But there are tons of great abdominal exercises, all with different benefits depending on your goals.
The sit up is such a specific motion, that it really has very limited benefits. Stabilization type exercises will benefit more. Some exercises will help with core strength alot, without being directly "ab" exercises. Heavy weighted squats are a great example of this.
If your goals are only to get a 6pack then you don't need ab exercises as much as you need a very disciplined diet supplemented with lots of cardio.
For 6 packs, good diet and being lean/athletic is all you need.
Also hanging knee lifts, leg lifts, and l-sits.
L-sit pull-ups are great for time efficiency. Those are mainly what I do for core these days, along with some squats/deadlifts, and it seems to be enough to maintain pretty strong abs.