Weight/Price not an issue?
Magpul PRS with a JPS SCS
Otherwise, price and exact usage will play a good role in this. An adjustable gas block is going to play a bigger role than your buffer system when it comes to felt recoil. A captured spring system such as the JP or Armaspec will cut down on the noise. If you really do want to smooth things out further, then go rifle length.
For getting an exceptionally soft shooting gun, look to what 3 gunners and competition shooters run - low mass BCG's and lightweight buffer/spring systems off an adjustable gas system that's putting back only as much as is needed to run the bolt. On the other end, for a fighting rifle, I'd want standard or slightly increased mass with something like a Geissele Super 42 or Vltor A5 system running off a Geissele gas block that's a bit over-gassed.
Both are opposite ends of the spectrum, but both will do their job very well. What you'll be using the gun for makes a fair bit of difference in what a good buffer setup will be.