I sounds like you're more interested in losing bellyfat than strengthing your core? The only way you're going to do this is eat less and/or burn more calories. You're not just going to pick a section of your body and burn the fat off by exercising that body part. Doesnt work that way.
If you are interested in building strenth...
Squats are the best exercise for your core, and as a consequence the most important exercise period. Deadlifts are the key lift after squats.
I like situps too, to the extent they don't hinder your progress on the above two lifts. Some people don't.
You'll lost a little beer belly this way I think. To finish kill off the blubber, you'll eventually have to spend some time restricting your calories and/or burning more calories.