Oooh, I got a few. When I was 6 or 7 or, I started building a space shuttle out of lego, as I was watching a lot of space programs at the time. It started fairly simple, the cargo hold, the cockpit, and so on. But over time I kept adding things to it, like sliding doors to separate the various sections, more stuff inside, and resized it to fit some figures in it. When all was said and done, it had sliding doors, two opening doors for it's bay, a robot arm, loads of stuff crammed inside it, and three big booster rockets. Probably the thing I was proud of the most was the fact that I made the nose of the shuttle out of regular bricks, but it still looked nice and smooth and round with the way I put it together. Another one was one of the Tripods from War of The Worlds. Getting the damn thing's long smooth legs to work wasn't easy, but eventually, it came through, and it stood about 45 cm tall. And there were other too, but I remember that those two were the best constructed, probably the best things I'd ever made.