Where would the best place be for an outpost on Mercury?

Well...believe it or not, water has been detected on Mercury near the polar regions by studying the radar reflections from the surface material. That means that of all the locations on Mercury, the place that offers the only natural source of water is at the north or south pole.

Humans need water so we would tend to favor those places in the solar system where this is freely available rather than having to bring it with us. So, I would probably put a colony near the north pole region, where the slanted rays of the sun would make it a naturally cooler place that in the hot equatorial zone. Being so close to the sun, you would also have a more severe problem in maintaining a comfortable temperature regime so again the polar regions would be favored.