No specific models or barrel lengths, but, considering the past 111 years:
Any list of the essential S&Ws HAS to include the M&P/Model 10. Not fancy, not rare, it just made the company in the 20th century.
Other essentials would be:
N-frame .357 - the first magnum
N-Frame .44 magnum - obvious
J-frame .38 - the ubiquitous pocket heater - carrying the company into the 21st century.
K-Frame .22LR - timeless classic.