Why the 'greatest' echo? It's the 'first' echo, that is of interest, not the greatest! The greatest echo can be a second echo from another reflection wall, that offers more area to reflect. Such a reflector will give a greater echo, which will arrive at a later time. But that's not your echo of interest!! It's he first echo, and this need not to have the largest amplitude.

So, all you need is a tiny bandpass filter in combination with a comparator, with correctly set threshold level. You don't need any ADC for this task. And if the signal is strong enough, as Mehdi told, then even the bandpass might be omitted. But take care, there are lots of parasitic ultrasonic emitters in your environment, which can interfere your detector. (You cannot notice them, because you cannot here them...). So, I would highly recommend to use this bandpass filter!!!

Again, please elaborate, what I mentioned in my last reply and post the results here. Then we can help you in designing a circuit.

Kai