I already know most people on here will answer with the Thunderbolt 1000t or 1003. In my opinion, the STL-10 running in attack mode is the scariest sounding siren. Just listen to this: http://www.youtube.com/watch?v=g5O9MsGNUY4
It almost reminds me of Silent Hill. I also think the HOR Super Sirex is pretty disturbing because of its oddball 7/10 ratio, but the 7 port STL-10 just takes it for me.