“The best” ab exercises are the ones that engage the entire core musculature. I think most of us have moved away from the tedious (and potentially unsafe) routine of directing clients to lie on the mat and crank out x number of crunches (or other often mindless spinal flexion moves). Obviously spinal flexion is required in life, hence our body’s design. I just don’t think it’s productive to spend excess time in our sessions doing that sort of thing.
The other point I think is worth making is the distinction between the abs and the rest of the core. Are you asking specifically and solely about the former, or do you mean the latter? A simple review of basic anatomy will reveal the interconnectedness of all these muscles and what you truly want to achieve.
Exercises such as stir the pot (on an exercise ball), bird dog (all 4’s on a mat), plank (front and side), etc.. are all great core strengtheners, while being gentle of the spine.
I also have moved away from ‘traditional’ ab exercises; however, I like to employ techniques in which people need to use the abs in the manner for which they were designed, i. e. as stabilizers.
A favorite of mine is the 1 Arm Chest Fly, which can be done lying on the floor, on a bench, on a Bosu, on a ball, on a foam roller …. you get the idea. I always cue people to be in a neutral pelvis, engage the core and have the objective not to move the body. As soon as the unilateral load is applied and moves outside the body, the client needs to deeply engage all core muscles in an effort not to move over to that side. What I like about this exercise is that people give me the feedback that they can ‘really feel it’ and thereby developing a better connection to core muscles.