I've done a lot of different ab workouts over the years, and I used to even do a short ab circuit every day in an attempt to get a shredded core (it didn't work). But no matter how many sit-ups I do, I find that the exercises that target my abs the most and leave them feeling sore as hell are compound movements that require full-body engagement.
Although compound lifts like deadlifts, renegade rows, and pull-ups fire up my abs, I find that nothing does the trick more than Animal Flow, a bodyweight-based movement system that combines all kinds of exercises from yoga, calisthenics, and breakdancing to gymnastics. Each Animal Flow exercise requires a tremendous amount of core strength, and there's no way you can do the exercises successfully without engaging your entire midsection, due to all the rotation, extension, and primal movement such as crawling or crouching.
I've been studying Animal Flow for a few months now, and I'll be a certified instructor in a few weeks. Nearly every one of my clients I introduce Animal Flow to tells me their abs are sore as hell the morning after working on these exercises for the first time. Trust me, even learning the basic movements — Crab, Beast, and Ape — will help you tone up the entirety of your core.
If you want to know how to do this workout or incorporate it more into your fitness routine, you can check out the Animal Flow Instagram page. You can learn how to do the basic movements as well as try out some flows that put all the exercises together. That's when you'll really feel your abs burn.