Leg raises and sit ups.
Try abs Inferno tb :-)
The best exercise for 6 pack is clean diet. Speaking of exercise I prefer Leg raises hanging on a bar.
Do at least 50-100 situps leg raises (left middle right) crunches of any sort for a day for a month or however long u want and take atleast two days of each week to rest
American Council on Exercise says the bicycle crunch, also called the cross body crunch, is the most effective abdominal exercise. Persistent multiple abdominal exercises and cardio exercise, over time, will reveal your abs.
First thing if u not about 12% or less body fat abs not happening the old stand by works sit ups leg raises have u been doing planks just may be the missing link. It took about 300 sit ups a day first time I had abs lost them in the 90s but got them back now at 56 years old doing about 100 sit ups every other day 3 sets of35 leg raises and 2 sets of
Planks I do planks and leg raises 5 days a week good luck
Leg presses and mountain climber