Some of the best shooting, best made, and accurate 22LR semi autos have got to be the Ruger Mark series. Although I have no direct experience, I hear the Browning Buckmark series are very similar. I highly recommend these as top notch 22LR pistols.

Now, in terms of the best and most similar fit to your S&W Sigma... maybe these guns don't make that cut. If you want the same grip, trigger pull, and sighting then the best you can do is get a conversion kit (if available) or get the same gun in a 22LR caliber (if available). Everything else is going to be a compromise of some sort. If you find yourself compromising too much.... then check out the Rugers or Brownings mentioned earlier. I don't think you can do any better in terms of quality, accuracy, and reliability.

The M&P-22 is a nice pistol, The Walther P22Q is a nice pistol. To me.Both are the best in there class....