One of my favourite models of all time is the Snotling Pump Wagon, not the new montrosity, but the original one from Citadel. What I liked about it most was the humour behind the model, you got the idea that the designer was down the pub when the idea came up.
The model initially came as a single decker with an expansion pack providing the top deck if needed. In later years the complete model was made available as a single kit.