Personally, I like the water plant the most.
Pros: Tunnels, pipes, storage rooms, water getaways (running away), a lot of hiding places, more landmark rooms (pyramid has roof and basement, this could have control room, roof, power room, water cleaning area, water dump area, storage room, etc.)
I really like the idea of something interactive, like the water, or maybe cutting the power/water flow, something along those lines.
Cons: Tons of pipes needed to be built, 'what would the main room for a water plant be?', outside (what is around the plant, background stuff, like steam chimney tower things)
It would also kind of be a pain to build.
