This is very individual as body types differ. For me, hard work hard physical labor such as yard work, lifting, perhaps building a wall with stone ... this was the type of work that got me a six pack, but at the same time I was losing weight for a health problem (Celiac) so my body fat got low.
Usually what I do is track circuits. Basically you sprint a 1/4mi or one lap at a track and then to 20-50 sit-ups/ crunches(don't over exert yourself other wise it just hurts your muscles not tones them). Do this pattern 4-8 times, also I like to add pushup and russians so as to keep it a rounded workout. This routine helps to increase stamina, core strength, metabolism, and cardiovascular health. Of course this won't privide immediate results but you should start to notice abs in 2-3 weeks(as long as this is done 3-4 times a week). Remember the longer you do a workout the more your body adapts to it, so remember to slowly increase the intensity or number of reps. Good luck!