The Colt Single Action Army all the way. Nothing else comes close to the mystique, myth and legend that surrounds the SAA. The M&P is proflic but few people lay awake at night thinking about them. If they want one, they just go buy one. Like it or not, movies and TV have always had a great influence on people. Just like stories by gunwriters like Skeeter Skelton and Elmer Keith. The Colt SAA was made legend, a little by history, a little by the famous company that produced it and a little by Hollywood. The SAA represents the Old West. It was made famous through countless western TV shows and movies, from the earliest silent films like "The Great Train Robbery" to the plethora of TV series like "Bonanza" to the remake of "3:10 to Yuma". Nothing else comes close. The model 29 has Dirty Harry, the Python has The Walking Dead and I don't remember the M&P ever getting much press at all.
The SAA has been in almost continuous production since 1873 and has been replicated by companies like Great Western and Uberti since the 1950's. If a shooter is fortunate enough to own one, it is likely the crown jewel of his collection.
The SAA is also probably the most adorned handgun ever made. No other handgun has been the subject of so much engraving, custom stocks, fancy finishes and other custom touches.
The SAA is the most written-about handgun ever. I have yet to see a book dedicated to the model 10, model 29 or Python. Yet my personal library is rife with books about the Colt SAA.
Sorry but nothing else on the list can come close to all that.