I'm always more impressed by feats of mechanical engineering than by electronics or software. This incredible model of a Soviet Hind 24 took took 15 years to complete and has more than 100,000 parts, many of them fabricated under a microscope! Check out the site for more photos and information.
"This helicopter has many authentic features including: linked controls (pedals that work in parallel motion), a battery powered motor that spins the main and tail rotor in the actual ratio of the original, adjustable tail rotor pitch, fabric covered stabilizer, landing gear with pneumatic rubber tires, compressable shocks, working lights, locks ,fans, aimable guns and titanium engine parts.
Complete accuracy is followed right down to the rivets and screws whose diameter and separation are kept to scale."