I posted this in another thread.
Metal sided exterior. Maintenance free (or nearly so aside from a yearly washing), fire resistant, and good looking. Same type of metal as a typical barn or shop. We just finished building a house like that and did a couple extras to make it more energy efficient. The metal siding requires purlins (2x4s mounted horizontally on the exterior studs) to attach the metal to. We covered the purlins with 1 inch rigid foam sheets and then screwed the metal over that. With 2x4 studs, 2x4 purlins, and combined with the 1 inch exterior rigid foam, we have the same thickness as a 2x6 wall with traditional OSB. But, our walls are MUCH higher R value. A typical house with 2x6 walls has R19 but about 20-30% of the wall surface is studs which are a thermal bridge for heat....so the true thermal performance of a traditional 2x6 wall is probably more like R14-15...less if it's not well air sealed. Covering any wall with rigid foam drastically cuts that thermal bridging down. We then used damp spray cellulose in the walls. The cellulose in a 5 inch wall cavity (3.5 inch 2x4 with 1.5 inches of 2x4 purlin) gives us a true R19 with good air sealing (damp spray cellulose doesn't allow nearly the air movement in the walls that fiberglass batts allow). Then, add the R4 for the rigid foam EPS and our main walls are at a true R23 and are also better air sealed.