Most sought after? The one up there at the very top of this page. I suspect that model (and that specific specimen, to narrow the range of opportunity) is likely to be the one most seen by thousands of pairs of yearning eyes with absolutely no chance of being acquired by 99+ percent of the people whose eyes fall upon it. If that isn't a really good definition of "sought after," I'm not sure what is.
OK, there are other more serious definitions. Sales volume speaks to appeal, so the guns that S&W produced in great numbers on a non-contract basis clearly deserve consideration for the title. Can we just stipulate that the .38 M&P (eventually the Model 10 and its descendants) is a hands-down winner based on this standard?
Sometimes a lack of popularity or demand at time of production leads to a short supply after the gun comes into its own. One definition of "sought after" might involve how much time the average would-be purchaser has to devote to the hunt before finding one to take home. This approach suggests that models with production runs of 5000 or fewer are going to be sought after, and if fewer than 1200-1500 were produced they will
really be sought after. Examples would be I or K frame target revolvers in .32 Long, short-run prewar specimens of guns that became more popular after the war, limited production varieties within larger contract production like the two-inch Victory, and so forth. In this group we can also add surviving military specimens from larger production runs that mostly ended up in the crusher after their service life was over.
There is a Goldilocks principle that might come into play. For most people, a lot of guns will be too small or too large, but some will be just right. Some chamberings will be too wimpy or too powerful, but one will be just right. Some sets of design features will be too simple or too busy or... and so on. On that basis, I'd have to give the nod to a .357 magnum with adjustable sights and a four-inch or shorter barrel on a K-frame. Does anyone recall if S&W ever made something like that?
Whatever answer to this question applies to any of us, it's a safe bet that some of us have no interest in guns that are items of fascination to others, and vice versa. I wouldn't have it any other way. A wise man once said something about differences of opinion and horse races, and that pretty much applies to gun desires as well.