I'm not actually sure what the largest and most complex LEGO set I've constructed has been. I'm pretty sure it was part of a set though... maybe the Millenium Falcon.
Yes, it's the largest official LEGO set ever, with something like 5000 pieces to it.
It also costs about 500USD... man, I wish I was rich :thumbup:
As far as my own LEGO creations go, the largest has to be my post-apocalyptic truck, which I did based on
the truck featured on the final scene of the game Full Throttle.
Sadly, I had to leave it at home when I moved out, so I can't get a picture of it... it's still there though, hopefully still in one piece.
Here's a drawing of it I did some time ago...
Imagine that, only it's done in LEGOs

It uses about a thousand parts... I never counted, but it's pretty heavy. (about 2 or 3 pounds maybe)
it's mostly done in LEGO Technic pieces, so there's a steering bar, rear suspension, and a number of cobbled together bits like cams and brackets that form the engine blocks (there are supposed to be 2 engines there), also there's a great deal of pieces from the underwater LEGO sets... so the truck is mostly yellow and black.
I had plans to build it's trailer, but I didn't have nearly enough parts to keep it proportional... The truck is about 30cm in length by 20cm in width, and 15cm high... the trailer would have measured nearly a meter in length
Cheers