Without a doubt, Nikola Tesla 1856 1943 a Serbian American physicist, inventor, electrical engineer, mechanical engineer and futurist best known for his contributions to the design of the modern alternating current (AC) electricity supply system is the most important 'Unappreciated Engineer' ever.
He held over 300 patents worldwide yet never received any official prize or award.
Inventor of alternating current in a practical form with his Tesla (induction) Coil, his work has made almost everything in our modern world possible. Thomas Edison and Guglielmo Marconi used his patents blatantly to achieve fame yet both received Nobel prizes, and Tesla died almost penniless in a hotel room where he lived.
On a topical note he was the designer and inventor of the first radio-controlled boat, so we have so much to appreciate from his genius.
'In 1898, Tesla demonstrated a radio-controlled boat (U.S. Patent 613,809 Method of an Apparatus for Controlling Mechanism of Moving Vehicle or Vehicles)'