What are the least powerful airplanes that ever flew?
There are a number of human-powered aircraft (list here).  For the Gossamer Albatross, we have


  In still air, the required power was on the order of 300 W (0.40 hp),
  though even mild turbulence made this figure rise rapidly.


As far as top speed:


  Allen completed the 22.2 mi (35.7 km) crossing in 2 hours and 49
  minutes, achieving a top speed of 18 mph (29 km/h) and an average
  altitude of 5 ft (1.5 m)