I'm trying to keep my own personal S&W loves and dreams in check while I answer your question. When I see the models you've gotten, the most obvious hole is a 39-2. Why? Well let me make my case:
1) The 39-2 is an iconic, historic, significant and noteworthy pistol in American firearms lore. So if the subject is "my collecting theme" and where to take it next, well this is the flagship semiautomatic pistol in the entire history of the S&W mark. This model is as much/more a part of the American law enforcement movement to semiautomatic pistols from revolvers as any gun in existence. (Hand in hand with the 59, arguably a variant of the Model 39 design.) So that's my "why collect a 39-2" angle.
2) Shootability! I look down your list and see a slew of beauties and fun guns. Add a 39-2 and you have a heckuva good chance of adding THE BEST single action trigger of anything in your list. The 39-2, by popular opinion, tends to feel fantastic in most shooter's hands. And the single action trigger tends to be better than all 3rd Gens that were made and it's an easy guarantee that it'll outclass all 2nd Gens save the 745.
3) Curb appeal! Look at the old school wood stocks, smooth bluing and slick single stack profile. These pistols are totally attractive.
4) Availability AND value -- S&W made like 347,000 of these over 12-plus years. They aren't scarce and the only "expensive" ones are time-capsule dead mint, unfired examples. A 39-2 in great shape isn't expensive and a beat-up one is dirt cheap. The only way the 39-2 can drain your gun fund is because you may end up wanting more and more of 'em!
5) Others pointed it out -- your collection shows no 1st Gens! If this is going to be a Smith & Wesson collection and not just a stack of guns then it's silly to have zero 1st Gens. 1st Gens are how and why every model in your list exists.
Oh sure, there are other 1-2-3rd Gens that I love, but if were in charge of this operation, we wouldn't discuss a single other model until this list shows a 39 or 39-2 in the first slot.
