Loudest horns I’ve ever heard would be the Federal/Standard 30A’s (aka “explosion proof” horns) that were in my elementary school prior to 1992. I’m sure that other AC horns like the Edwards Fire Horn, Simplex 40x0, and so forth give them a run for their money, but those are the only ones I’ve heard.
As far as electronics go, the Faraday MTL’s at the NYC Port Authority Bus Terminal get my vote. The U-MMT’s where I work now don’t seem nearly as loud and obnoxious, so I guess it’s a matter of the setting they use.
For speakers, I’d probably say the Wheelock bullhorn-type ones at my college stadium, which played the standard modern Simplex slow whoop. I didn’t hear them right from in there, but the fact that I could literally hear them from a mile away should say something.