If you want the absolute Classic, it's without any doubt the 4 inch M&P or later the model 10. If you want the pinnacle, IMO that would be the 3 1/2 inch RM or the later model 27.

Now for the good news/bad news side of this question. In it's long history S&W has made so many splendid revolvers that there will be considerable debate on this issue. Bad news is that the simple fact is that in terms of numbers and useability nothing better defines the S&W revolver line better than the M&P or model 10.