I think the best invention is the bicycle- a phenomenal piece of engineering. How did the first person who made one figure out how to ride it? The bicycle contains lots of chains, wheels, pressure and complicated physics. What about when you turn a corner? Or fly down a hill at speed? Or change a gear? There is friction in the wheels, spokes to spread the weight of the cyclist, cranks in the pedals to increase the force, big wheels to enhance the speed and a strong frame to distribute our weight. Wow.
Second to that has to be the aeroplane! Tonnes of metal in the sky using wind flow beneath a wing. Crazy.