The "softest shooting" .45 ACP will ALWAYS be a REVOLVER! When you drop the .45 ACP into a revolver with a fixed breech and no need to cycle a slide to function, the recoil becomes amazingly mild. The .45 ACP after all isn't really such a powerful cartridge...around 350 lb-ft of KE outside of specialty loads. In a 38-45 ounce revolver the .45 ACP is a pure pussycat.
A semiautomatic .45 ACP "kicks" because the relatively heavy slide mass moves rearward, strikes the frame limit, causing the pistol to rotate up in the hand. A 12-15 ounce slide produces a lot of momentum and this is exactly how and why recoil operated firearms work.
Recoil is also highly subjective. The fact is, the .45 ACP really has no more, nor less recoil than a 9mm with commensurately weighted and sprung slides in approximately equal frame weights, because both rely on the relatively high mass of the slide to make the gun function. Technically, GLOCKS should kick harder than everything else because most of their mass is concentrated in the upper unit....with relatively heavy slides compared to the 1911, but, Glock's striker system also allows the shooter's hand to sit very close to boreline as opposed to SIG and H&K, and really, everyone else. This reduces the rotational force applied to the wrist resulting in lower perceived recoil.
Once you remove rotation as a factor in perceived recoil, "kick" seems a lot less punishing to most people. For example, my S&W M-500 has very little rotation during recoil due to the excellent muzzle brake, yet it does kick hard directly back into the palm, yet lacking the massive, torquing rotation of most hard-kicking magnums, the overall sensation is fairly mild.
The same is true of my Glock .460 Rowland with 6.61" barrel, long slide, and LWD recoil compensator. The gun has little perceived kick due to negligible rotation, combined with the wide, polymer grip frame spreading out the direct pushback. Come to think about it, when I try to shoot .45 ACP from my .460 Rowland with the heavy slide and 24# spring, the cases barely "dribble" out of the ejection port (some not at all) with basically zero "recoil!"