Crunches, sit ups, planks (the real ones, not this craze crap that's going around), reverse crunches, double crunches (upper and lower at the same time), and don't be afraid to add weights to your chest or feet to challenge yourself more. A lean diet is necessary for optimum definition. Good luck!
Abdominal serrations are not a measure of the strength of your core, but the leanness of your body. Everyone has abdominal muscles right now to some degree, the trick is getting lean enough to see them.
I run sprints. All out, fast as you can 60-100 yard sprints. I despise crunches because the stress on my neck and back is uncomfortable. A proper diet is just as important. Also, for every lift, pull or push you do, concentrate on flexing the abs. Drink plenty of water. Really, do what you feel provides you the most benefit. Try different things. Looks as though your already doing something that works for you, and whatever you do, apply yourself 100%.