First of all, I (and many others on this board) will probably tell you that the best core workout is heavy deadlifts. When you are deadlifting heavy weights, if you don't have a strong core, that weight's not going to go anywhere!
A few specific core exercises that you might want to try.
Decline board iso holds
Decline board russian twists
But if I had to choose one it would be the overhead squat. I overhead squat every workout, usually just with light weights as a warmup, but I also work it in with heavier weights. Simply put, overhead squatting will work every single muscle in your body. I've definitly seen an increase in my deadlift since I've started doing ohs's.