If you want to make your core strong I recommend doing plank, hollow body position, you can try the dragon flag if you are at a more advanced level. If your goal is to see your abs better, keep in mind that diet plays a key role in this.
Hold plank for 30s and rest 1min. Repeat 5 times
Same with hollow body position.
You can rest more if you need.
Remember to keep a good form in your exercises