Mine would be my Taurus .44 Magnum. With the short, ported barrel...factory .44 Magnum loads always get someone's attention. At the range lately, I have been burning through my light Special reloads. Not a lot of bang like the Magnums.
My T/C Contender with 14" barrel and muzzle tamer brake shooting full house .45-70 cartridges. Guys wearing ear protection at the other end of the outdoor rifle range stop and look. I think if you shot it inside a house it would knock plaster off the walls. I call it my Manly Handgun.
It's gotta be my .375 CheyTac with muzzle brake. I shot it at Cresap once, with those partitions between the benches, and it felt like someone reached up into my head and punched me in my sinuses (deflected pressure from muzzle brake off partitions).
I'd have to say my S&W 460 is my loudest, I think it's even louder than my 375 H&H...didn't think that was possible, but that 460 will make you deaf for weeks if you're not wearing hearing protection.