the first consideration is decide which caliber/calibers you want to focus on....since you already have the 6906 I am thinking 9mm will be one....and it probably should be in everybodys collection...then..decide the fire control platform that you are more comfortable with TDA,DAO,or SAO....makes the hunt for 3rd. gens a lot easier.....myself I limit my choices to TDA 9mm and .45acp.......so when I happen onto a fine .40sw or 10mm.....I just grit my teeth...wipe the perspiration from my brow and move on.....and my wallet appreciates it.......and you are right....Harry Callahan did say "a man needs to know his limitations"..........and I do

...........and as the one gun that should be part of a 3 rd. gen collection........well depending on your caliber choice......if .45acp is in you future....a 4506 is pretty much mandatory .........pretty sure it's state law
It has always fascinated me how gun collectors and collections vary. Different strokes for different folks I guess. As much as we are alike, we are different. Practical matters can be significant too. Lord knows we are not all wealthy. We don't all live in areas and under political regimes that are conducive to successful gun collecting. Some of us bring history to the table that causes us to focus in one direction vs. another. Sometimes it is just sheer personal preferences or chance opportunities. Some like old, some like new. The list of variables is almost endless. I could talk about it for hours.
My own story can be summed up like this: Man who grew up in the shadow of S&W's Springfield plant starts off collecting mainly S&W revolvers and a few early generation semi-autos.

Same man unfortunately goes Rip Van Winkle for 25+ years and wakes up to find pistols are now all plastic!

Man wonders what the Hell happened?

Man sets out to figure out all that he missed while asleep and to get his S&W collection back on track.
I've said it before: The evolution of the S&W semi-auto interests me just as much as (and sometimes even more than) the guns themselves. I'm not necessarily out to own the absolute best of the S&W semi-autos. Rather, I am out to acquire the ones that best illustrate the engineering & business evolution of S&W semi-autos into what they are today, including the painful transition from all-metal to plastic. Since much (most? all?) of that S&W semi-auto history was driven by the law enforcement market, that is my first interest. What did cops want? What did cops buy? What worked? What didn't?
In that regard, I don't see my collection having a limited caliber focus nor a limited firing platform focus. While I have always preferred DA/SA for my own personal use, I want to own examples of each caliber and each platform.

And Lord knows that I'd
love to own a 10mm 3rd Gen someday.
In short, what S&W gun models best define the evolution of the S&W semi-auto? For the most part, I would think they would also be the most popular/successful/mainstream models, but not always.
The 4506 is absolutely on my want list.

Quite frankly, I thought it was going to be my very first 3rd Gen. Only scarcity (within MA) and very high asking prices have probably pushed that back to my third or fourth likely 3rd Gen acquisition.
