Anapannasati - Why the breath?
Seems that moderators are expecting a particular style in these answers, and so deleted my initial answer. I'll rephrase...

The reason that the breath is the most important meditation object is that unlike other objects they are always available. For example, you might forget your TM mantra or misplace your mandala. Other objects of meditation, such as decaying corpses, aren't usually freely available in the UK either :-)