Best sensor for breathing activity?
Probably a thermistor to look at the temperature change between inhaled and exhaled breath would be easiest.  If you're really interested in flow, then a thermistor in self-heating mode to be cooled by inhales and exhales.  the latter is trickier, as inhaled and exhaled breath will be two different temps, so the flow sensitivity for each phase will probably be different.

Moving up the line, you can also look at chest inflation using skin plethysmography, which is essentially the broadcasting of a small sin wave, and looking at the reception at a distant location on the chest.  The sine wave should really be isolated.