Easily this would be the siren in my hometown, a Cicca R3P, aka the loudest French air raid siren ever made (owing to the extremely tight tolerances on the rotor), back when it was still tested.
Considering I lived around 500 meters away from it back then 
The 2nd loudest siren I ever heard, this time was in Lamorlaye, and this time around it is a Constructions Électriques du Nord (CEN) type NP3S, while it wasn’t as loud as the R3P, it still packed a real wallop:
