Naively I would assume the closest you can get to an impulse function, aka a Dirac delta function, because it contains all frequencies at once. So, as loud and short of a sound as possible. This should give you the most information about your surroundings from listening to the echo, because different frequencies can reflect differently in a room. Also because mathematically an impulse convolved with a function returns the function, so an impulse gives you the most neutral echo with which to compare.