In my case this question poses an interesting theoretical problem. I don't know how to identify my "first" S&W because I bought perhaps 15 of them at once.
A friend of a friend passed away and they had to liquidate his collection. I purchased maybe 25 guns...a large percentage of which were very nice S&W revolvers. So in my case I got a whole bunch of Smiths at one time.
I guess if I had to pick which came "first" it would be the one that sparked the most interest and got me into the auction. It's a 629 .44 mag with the 8 3/8 inch barrel. It's a no-dash 1980 vintage gun and I really, really wanted it badly the moment I saw it. At the time the only revolver I owned was a Ruger Blackhawk in .41 mag.
Then I picked up another 629 in .44 with a shorter barrel and non-fluted cylinder, then a model 29 that has since become my favorite in the whole collection. There were just a mess of them...a bunch of 45 colts, a .22 revolver, a .44 special...it was a fun week. Heck...it's been a fun year since then.