I prefer No Theme. (I chose other.) Mainly because it doesn't hinder anyone's imagination, and usually it's an unspoken agreement to not do future when people are building castle's and what not, or to do anything else that is quite rediculous.
However, my favorite theme of all time, is the "Perpetual Night" Theme. Basically my friends and I did this on DoB5 Ice, and had it time locked on midnight, and had the entire map covered with one weather of snow. And basically there were typically a few guys trying to find a way to end this perpetual darkness/snow, and then there were the guys trying to snuff out any light that humanity may have had in succeeding in such a goal. Then there were the guys in between, not really caring for either side, just accepting the darkness and trying to live another day.