Let me start by saying I own at least one of all of them.
First: The model 27 is the King of the S&W line (in MHO) Well built, accurate and the gun that started it all. N-frame gun, large and heavy.
Second: The 686 Everything you could ask for in a 357. Durable, again accurate, Lite enough to carry every day and tough enough to shoot any reasonable load every day, all day. L-frame gun Smaller than the N but heavier than a K-frame
Third: 66/19, I lump these together because except for the stainless vs. carbon steel these are the same gun. Again they are accurate, easy to carry and great to shoot and to my mind the near perfect platform for an everyday carry, 357. I'm not going to get into the lite bullet discussion because it is often discussed and cussed. K-frame guns. Lite but strong enough to shoot 357 and still be reasonably durable.
As I said, I own 1 or more of all four plus a couple of 28s(which are just 27s without the fine finish) And while I love my K-frames, If I could only own 1 357 platform it would be one of my 686 guns.