Before yesterday, I would say the Rathaus*. An extra -25% maintenance? For a UB, you can't beat that with a stick. But yesterday, I did something I've never done before: I enacted the slavery labor civic. I fought with myself over the prospect employing such a reprehensible policy. The dishonor and feelings of being of less of a man have prevented me using slavery since Vanilla CivIV. However, my urge to become a better player finely overcame my ethical qualms, so I swallowed my pride and I stared whipping away. I'm enjoying it so much that next game I'm going to stoop even lower and play as that detestable Monty b@st@rd, just so that I can use the Sacrificial Alter!
As an historic sidebar: In Augsburg, Germany there's an awesome Rathaus. Inside it there's a HUGE main room that has 24 tons of gold leaf on the ceiling. The building is tall, also. It has a tower attached, that from the top of which, you can see the entire city and all of the surrounding countryside. After reading the UB and remembering my visits to it, I'd say that it is an excellent choice for a civ's UB.