Ok, I'll bite. I voted for 25mm (as in CV25/4). It's a focal length that's very well suited for scale focusing, but can still be used without every picture turning out a gimmick because of perspective distortion. What's more, it's a very high contrast lens, which means it can bring out definition in very low contrast areas like when you're shooting in the middle of the night..
Attachement: CV25 8sec at f4 on Fuji Pro160C..