Not lego, but i absolutely loved K-nex as a kid. Got the biggest boxed set of it for christmas one year,
Spent days building this, it was literally taller than me at the time i was so chuffed when i finally finished it and it worked when you switched the motor on. Best bit about it, was that there was still loads of pieces left over so you could customise the design, add more stuff to it or build other things without having to dismantle the whole thing for pieces.
I remember watching that inside Lego documentary a while back and you have to give huge credit to them for turning around a company on the brink of going out of business to making it not only relevant again in the digital age but more profitable than it's probably ever been.