Your body fat percentage will determine how visible they will be. Definitely add some cardio (since all it takes is a thin layer of fat to keep your abs from showing). There's not really one "best" exercise since you'll want to do a combination of different ones for more effectiveness. Sit-ups are good, but also add in leg raises and some oblique exercises. Leg raises to target the lower area of your abs. Standing oblique crunches and Russian twists for your obliques. You can use anything at home for resistance if you don't have any weights. Finish off with some front and side planks. You can do them in sets of reps or timer based, but assuming you're new at this, then typically 4 sets of 12-15 reps each with 1 minute planks are pretty standard. Otherwise modify to whichever works best for you.
That's typically a standard ab workout. There's other exercises and variations you can try also, but definitely make sure it's a combination that targets those different areas. But like the other guy said, full body workouts will be most effective. The real challenges, however, will be eating right and consistency so good luck!