The Dragon Flag is arguably the best ab exercise and probably the most difficult one you can do. If you can do these in “good” form, you have core strength without a doubt. Popularized by Bruce Lee and sometimes referred to as the Rocky Ab Workout, this is one of the top three best ab bodyweight exercises you can do (the other two being the full hanging leg raise and the abs wheel roll out).
This should not be attempted by somebody who is just beginning. This is an advanced core exercise. If you want to start building your ab core strength slowly, I suggest the classic abdominal plank which is the simpliest type of ab exercise to master or the reverse crunch.