i don't believe in doing crunches because it shortens your hip flexors. same with hanging leg raises. a six pack is made in the kitchen, period. you can have an amazing six pack but be weak as fuck. you can also have 20% bf so no six pack, but have excellent core strength. now when it comes to doing exercises to get that core strength, i say planks, also lie on your back and hold your feet 6 inches above the ground. now from this position you can do hello dollies or flutterkicks. also from the 6" position move your legs around in a circular motion. no need to contract your abs at all in my opinion. the only thing i do contract are my obliques, once in a blue moon i'll get on the machine that allows me to hold plate in one hand and sort of crunch my obliques. deadlifts strengthen my lowerback and obliques though, and your entire core is under alot of pressure stabilizing that weight on squats.