Hmm, the best all around is still a bit of a compromise, as the best for a given situation depends on characteristics that no currently produced textile can achieve as a whole. In winter, nothing beats my rivers west fleece. While not a poncho option, their clothing line is waterproof, breathable, and quite warm. Their blankets make darned good tarps. In the big heat of the baghdad, the fleece idea won't work, and nylon doesn't seem to like the sand one whit, so reinforced film materials for tarping off and breathable synthetics for clothing. In between the extremes, I would tend more towards the waterproof and the slightly insulated stuff. Heatsheets is probably the best compromise all around material I could imagine for use as a poncho or as a tarp. Tyvek would be a close second, and ripstop lightweight nylon takes third maybe.

Most of the milsurp I've worked with seems too much of a compromise to me.