There was a similar thread here not too long ago and while everybody likes the 357 and 44 Magnums, I believe the consensus was that the utilitarian Military & Police (later the Model 10), with something like 7 million produced, was the hands-down winner as far as what most people imagine when they think Smith & Wesson.

Edit (from Wikipedia): Boorman, Dean K., The History of Smith & Wesson Firearms (2002), p. 46: "The .38 in Military and Police Model 10 has historically been the mainstay of the Smith & Wesson Company, with some 6,000,000 of this general type produced to date. It has been described as the most successful handgun of all time, and the most popular centerfire revolver of the 20th Century."