What types of ab exercises work for getting a six pack?

Abs/core+cardio. There are endless varieties of crunches, planks, core exercises, pilates, situps, isometrics, etc. You can do to build your abs; you will need to build up no. Of sets, reps that you do to build abs. No matter how many crunches you do, if you ignore overall fitness/cardio/aerobic exercise to reduce body fat, your massive abs will remain hidden under belly fat tissue. Good luck; consider a trainer!