A belt for each the X and Y axis. This is the simplest possible arrangement. Unfortunatly, with a stacked-XY cube-style printer this means that one of the axis's motors moves with the other axis. This adds a lot of weight, reduces acceleration and speeds, and adds size.
This unusual setup was suggested to me by multiple experts. CoreXY uses a pair of crossed belts driven by a pair of stationary motors at the back of the frame. Turning either motor on it's own results in diagonal motion, meaning that both must be driven to achieve linear motion. Since the motors are stationary, this allows for lower inertia (faster motion), more rigid frames, and more compact mechanisms at the cost of a more complex belt system.
Relatively new, HBot uses a single very long belt. Otherwise like CoreXY, HBot carries all the advantages of coreXY without as much complexity. The only added downside is that untill recently very long high quality belts were difficult to affordably source.