I disagree with the Model 10 for this category as my definition, for all that's worth, of "classic" would be something that was made in enough numbers to be famous and quickly associated with the company, yet enough of a ground breaking advent in gun history that it was never confused with the products of other companies. Often copied by others though. Sure, the M&P (M-10) was made in huge numbers forever but it was all too similar to the Colts of the day.

It is an easier question when applied to others:

Ruger: the Blackhawk
Colt: the SAA
Colt, also ran: the 1911 (one could esily argue the SAA and the 1911 in reverse rolls here)
Colt, DA: the Python
Winchester: the Model 70
Old Winchester: the 1873 lever-action

I still think the S&W Classics were the 357 Magnum, the 44 Magnum, and the Combat Masterpiece. All highly sought-after and often copied originals that sold more than S&W could make in their eras. Now they are all highly respected icons of the company.