My favorite J frame is the Model 60 I bought in 1966 prior to my first visit to Viet Nam. But DAMN S&W keeps introducing more and more great J frames. My new 632 3" in 327 Magnum has really impressed me with it's versatile chambering. And I just bought a 43c in 22LR that is a true fun gun. There are so many great J frames both new and old it's hard to claim just one as the favorite. Nostalgia aside, if today I had to pick just one it would be the 632.
Another vote for the 649. All steel easy to shoot easy to carry. Snag resistant hammer but still have the option to shoot single action. My most commonly carried weapon.
As said, it's hard to go wrong with any J-frame; just beware of the gimmicks. Stick with steel or aluminum alloy, and you've got a gun that will be around for generations. Ultra-light scandium and titanium are well & good, but you pay with every shot! Another recommendation to stick with .38s; the .357 is just too brutal in these tiny packages.