I get asked all the time, "Alex whats your BEST or ABSOLUTELY FAVORITE CORE WORKOUT??"
The best answer I can give personally is.. get yourself some GRAVITY BOOTS.
These things are worth more then there weight in gold.
These boots were initially attended for Decompression purposes. Imagine the amount of compression your body goes thru day to day. The weight of gravity on you, carrying a briefcase, backpack, ect. Your spinal column is so compressed daily that it starts to lose moisture messing with your natural spine shape, causing discomfort. So going vertical upside down and letting the gravity work as a positive on your body allows for full decompression allowing some moisture to get back into the spinal column, helping with posture and back pain. The other strong plus is getting blood and oxygen into the brain this helps people that get headaches often. I personally think it helps me with having clearer thoughts, its not proven but I am just stating what I went thru. I've also heard things like helping with hair growth, vision and growing in height.
So for about 5 mins a day (I used to do them before bed) I would decompression, clear my mind and rid of my back pain.
But we are talking ABS here so! I would clip on the bar, let go and keep my hands just touching my forehead (no assistants from my arms) and crunch straight up and back down. Its not EASY at first but like anything else YOU get stronger with each workout. This workout will not only make your core super strong it will hit the big bellies of your abdominal muscles, giving you a deeper valley in-between each muscle which will cause a deeper shadow giving you a better appearance.
That being said the gravity boots I have are alittle pricer then the others but they are one of the best investments I ever made and if your serious about your health and you really want those type of abs that really pop there is nothing better. 
I am thinking about making a video using these boots, I get alot of interest on these boots in the gym so why not make an instructional video showing a ton of workouts that can be down with them. I think this brand is the best versus those cheap versions they offer. If your interested I am going to post a link for the same brand gravity boots I have.
Thanks for reading, if you want to see more post like this one FOLLOW me! and please RESTEEM and UPLIKE!!
Alex T - ABS.Co