Which is the best military helicopter in the world?

There is no answer to this question, as it is like asking "Which is the best car in the world?" It all depends on what you want to use that helicopter for, and no helicopter can be very good at more than one general area.

In the same way that a Formula One racecar, semi-tractor trailer, luxury sedan, and cargo van all Excel at what they do, and generally stink at what the others do, the same goes for helicopters.

Please re-ask the question in a more specific manner as to what mission (i.e. purpose) you want the helicopter to Excel at.