From a military perspective, the helicopter fulfills a variety of roles for all branches of service. There are dedicated platforms developed for specific battlefield requirements including form of attack, transport, observation, search & rescue, special forces insertion/extraction, anti-submarine / anti-ship warfare and replenishment. In any role, the helicopter is prized for its landing/take-off footprint and ability to reach areas that neither vehicles nor fixed-wing aircraft can reach. Helicopter systems on order heading into the new year are NOT taken into account.
Helicopter strength data through 2017.
Sources: CIA.gov, CIA World Factbook, wikipedia.com, public domain print and media sources and user contributions. Some values may be estimated when official sources are lacking.