Nelie beat me to it!
If by "work" you mean "remove belly fat", then there's no one exercise that will do that, because you can't spot lose fat.
The only thing that will help you lose belly fat is cardio and diet.
Now, you can work the abs, back, and core muscles so that as you lose the fat, your belly looks flatter and trimmer - and all the old standbys are good for that. Crunches. Planks. Any core work ... will help build those muscles to show off as the fat goes away.