“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.