My first was a hole I dug into the beach that I gave a couple rooms. It wasn't that great.
My favorite one was this sea-side villa. You had to go up some stairs to the entrance door. There was a double hallway that extended around a 7x7 courtyard with grass and a fountain in the center. The hallways led to the kitchen and work room on either side of the building. Both rooms had a door leading to the living room with a fireplace, book shelves, a stairway up to my room and a stairway down to the basement. In the basement there was a hall that led under the courtyard; the fountain was connected to an underground pool, there was glass at the bottom of the fountain to let light into the pool. The hall also diverged into a storage area that circled the pool, there were chests for every material I might come across. I had a harbor outside and was building a train station under the basement, it was pretty incredible.
I explored a ton of the world around the building before the 1.8 update. Is there a way I could copy the chunk with that house and paste it into a new world? I'd like to have a chance for villages to spawn somewhere closer to it. As is it would take me a 20 minute hike in any direction to get to new land generation.