Check out Livestrong's website for "The 41 Hardest Ab Exercises"
When it comes to core, make sure you target it in multiple ways. It doesn't just come from doing crunches. It comes from functional, total-body movement too. Deadlifts and movements involving full hip-extension also work your core.
Make sure when you do just core, you do it as a warmup before lifting, or running/cardio. Core serves to correct your posture and support your spine. So it's good to activate the abs before you run or do strength training because it will help wake up your stabilizers and entire core and in turn, support you through functional movement patterns. Think of the core as the body's powerhouse- the mitochondria of the body.
You can't out-crunch a bad diet. The secret to abs is to lower your body fat. And you should never have just an "ab workout," always incorporate it into routines with strength, running. Work it from all angles!