This is a tough one!
1. Lionel sawmill is a great one. Visitors always want to see inside and understand how the logs turn into boards.
2. MTH fire station. My four year old can't get enough of this one and I can't either. Watching the fireman slide down the pole, the flashing lights, the clanging bell, the garage door opening and the fire engine driving out to answer the call. So cool!
3. Lionel gantry crane. I don't have this one, but it is on the short list if they re-release a TMCC version. The video posted above has me hooked!
4. Lionel lift bridge. This is just fun from the shear size of the beast. Visitors are amazed to see it rise up all on its own.
5. Lionel TMCC crane car and boom car. Not sure if this fits in with your definition, but it sure is fun to play with.
Since the question was favorite I guess I have to choose one.
Lionel sawmill. Still a classic, clever accessory that interacts well with the operating log car.