Extreme core workout demonstrating some of the hardest core exercises out there. Finishing the workout with several easier ones!
You may not be able to start doing all of these right away, but the easier exercises demonstrated in the end of the workout should be doable by people who are reasonably fit.
The workout takes care of both upper and lower abdominals, obliques and it also develops heavily the hip flexors.
You can expect to see great gains in the strength, size and definition of the abs. The strength will surely transfer to better movement and athletic prowess.
L-Sit Work

L-sit is one of the basic gymnastic skills that almost anyone can learn. The basic form is to hold the legs straight together (your legs don't need to be spread straddle like in the picture).
The L-sit exercise trains the abs but also the triceps and hip flexors need to work rigorously.
How to learn the L-sit? - Parallel bars are the easiest tool, but you can also try to practice L-sits on your palms or on your knuckles.
In the beginning, just learn to raise yourself off the floor. Your both legs can be completely bent. Then, straighten just one leg and keep the other leg bent.
After you have practiced this enough with both legs, you can try to straighten both legs. It will burn but it will be worth it!
Supplementary exercises that will almost directly improve the L-sit are: hanging leg raises and jackknives (check our another core workout for this).
Tuck Planche to L-Sit

In the tuck planche to L-sit you first take the basic L-sit position and then pull your legs under to a tuck planche. Your arms stay straight all the time!
How to learn the tuck planche? - You just need to get stronger and fitter, then just try to push yourself off the floor by leaning forward. Pseudo planche push ups will help with the lean.
Hold both positions, the tuck planche and L-sit for 1-3 seconds and then do another repetition.
Both the tuck planche and L-sit will work the abs, but the transition between these two is the key element that works the abs, shoulders and triceps the most.
Check the video for the correct illustration.
Hip Rotations on a Gym Ball

This is an interesting exercise we learned from the MMA fighter George St-Pierre.
In the hip rotations on a gym ball, you balance your lower body on the gym ball with just one leg and then move the other leg side-to-side by rotating your trunk and hips.
The rotation will work your hips, abs and obliques. Use your arms for support!
You can do the hip rotations on a gym ball for 3 to 5 sets and 5 to 10 repetitions per side.
Kettlebell Rotations

This is a phenomenal rotational drill for the whole body. The movement trains the hips, obliques and improves the flexibility of your upper body.
You can also use dumbbells if you don't have access to kettlebells. Start light (even light as 2.5 to 5kg which is 5 to 10 lbs) and work your way up.
The arms can stay relatively relaxed, because you should aim to create the rotation from your hips and midsection, not from the arms.
You can do this for 3 to 5 sets and 10 to 20 repetitions per set.
Train hard, stay safe.