It all depends on your Level of Fitness, and Overall Strength.
I like Hanging Knee and Leg Raises with a Twist at the Top.
When done correctly, it's the most bang for the amount of work you put into it.
There's the Old School ~ Seated Twist with a Bar or a Broom Stick will also work.
Just remember to Start in the Middle and Work to the One Side Only, then to the Other Side, always Stopping in the Middle.
Don't Rotate Back and Forth, using Momentum to carry you.
Then you hold a Plate in one hand, and do Side Bends.
But don't let your Hips move out.
Just bend to the side with the Weight, without moving your Hips, then Squeeze Hard to the other side.
Then there's Touch Work done with a High Pulley Cable ~ Side Cable Bends.
Grab the High Pulley with a Slightly Bent Arm, and Crunch to the Side.
These you can vary your Stance, from Standing Side Ways to the Cable, to moving slightly inward.
These are High Rep finishers.
But you need the Low Body Fat Percentage, or you won't see much from these.
The Hanging Knee and Leg Raises are the place to start....................... JP
P.S.
There's Ton's more exercises that you can find on YouTube, to see how they are done.
Find a couple that fit your situation, and take at least a couple days a week to Kill It.