Tesla's inventions are of course immense... but I was always amazed by the inventions of Joseph Henry. He is the guy who discovered magnetic self induction, although I think Farady is credited with it... In testing his theories about self induction, he had to make a coil. Back in his days wire didnt have insulation, so he had to make his own insulation for his conductors. He took one of his wifes silk coats, stripped the silk strands off of it and by hand wound silk around his conductors that he used in a coil. That just blows me away that the guy would go through all that trouble, basically for the hell of it; to see if it would work. Amazing.
His induction motors, and coils to me are some of the most important inventions in the electrical field, as are the principles behind them.