The Lockheed Hercules is truely the greatest military aircraft in the past 100 years.  With its design is has proven itself in many different roles and uses in combat, combat support, logistical, and humanitarian environments.  What other aircraft  is still in service today that came from a design from the 1950's and looking to still be in use in the future?  We keep modifying it and it only get better with age.

Adaptive Aerospace Corporation
 

Agree on choice of aircraft, but there are two other aircraft with equal longevity - Bear and B52 :-)

...... And the near contemporary U-2 ? ..... : - )