Tough question! My top three is the best I can do:
#1 3.5" 357 Magnum (Model 27)
#2 6.5" 44 Magnum (Model 29)
#3 4" Combat Magnum (Model 19)

These are the great signature Smiths that are never confused with Colts or Rugers. Classic and legendary all. All made a huge impact.