so far for me it has been a 357 magnum. fired at an indoor range, a few full power rounds can make your teeth hurt, don't even want to think about anything more powerful. Had a Glock 32C that sure was a bit loud as well.
Any handgun loaded with 2400 powder &/or fired from inside a car. My old Ruger 3" hybraported SP101 357 mag was my, make people at the range move away gun. I no longer SHOOT WITHOUT EAR PROTECTION if I can help it. Damage has already been done, but not as bad as others, & I don't want it to get worse.
I agree with the others who have proclaimed the Ruger .30 carbine Blackhawk the loudest and most "ear splitting" to shoot. They are very accurate but, I have owned and sold two of them simply because they lost their charm as a result of the decibel level!
There's a difference for me between loud and sharp. I don't seem to mind my 460 or 629 ( they're both loud ), but not very sharp. I have a little Walther P22 that has such a sharp report that it is painful to anyone nearby without ear protection.
Loudest I've fired is my 5" .460 V, the shorter barrel is louder than my buddy's 8 3/8" XVR.
We shoot the same 300gr. handloads in both revolvers, (as my buddy do reload and I don't, yet...).
Both have the compensator holes blocked off with a ss sleeve.
I also enjoy the shockwave when firing it, it's a rush.
The loudest handgun I ever fired was a Taurus Raging Bull in .454 Casull. There was a tremendous muzzle blast and the shock wave disturbed people 8 shooting lanes away.