Ok, first this post is not to incite a riot among keyboard enthusiasts. Neither is it a this brand is better than X. We're talking about the model M only.
That being said, the IBM model M has a long lineage and I'm sure there were modifications and changes along the way.
We'll first split this discussion into two categories.
The full size IBM Model M and the spacesaver Model M (SSK).
Of these which vintage/model/part number had the best mechanical/electrical design and overall quality? Perhaps manufacturing standards were tight enough that they all perform the same. Maybe Lexmark let things slip one year, maybe there was a year that they skimped on plastic and one is weaker and has a tendency to fail more often. This is what I want to vet out.
If you were to want 1 keyboard that you would use every day which one would it be. Rarity doesn't necessarily equal superiority.
Why do I care? I like a challenge, I'm going to hunt down this keyboard, restore and use it.
Among all Model M keyboards? I will say that essentially, they feel the same across the board to use. That being said, each keyboard feels unique to itself. It really depends on the condition of the keyboard. If they are all new, they would feel the same, right?
Condition is a serious factor here. You can get a "NIB" SSK or 101 and it could be entirely unusable because the plastic rivets have completely sprung from the steel backing plate.
If that is the case, you need to have it bolt-modded. Once you have a bolt-mod done, your Model M will most likely feel and perform like new.
Arguably, the first-generation Model M had a heavier galvanized steel backing plate that has a goldish-rainbow color to it. It is slightly heavier, and the older clam shell cases that the mechanism sits in are essentially the same.
Does it make it better electrically? I don't think so. The controllers have changed a bit over the years, getting smaller and smaller in form, but the function is the same.
The good news, up until the 3rd generation of Model M, you can swap the controllers easily between keyboards due to the keyboard matrix being the same, and the pinouts being the same on all the controllers along the line. (There is more to this than what I am saying, but I am generalizing)
I have an SSK, is it better? I don't think so. It is compact and very usable. I guess if you feel you can use a keyboard without a built-in numpad, it's a benefit. Just don't activate the numlock on the SSK, unless you plan on using that layer for a numpad and unlock it again. (I find that very annoying)
The good news is I have a LED indicator for both num lock and scroll lock being on, so I can switch it off and know it is off.
The keyboard I use as a daily driver is my 1392595 terminal board I converted to use with my PC. I love it! It looks great and does the job.
http://imgur.com/a/O7ayYDoes it make it any better than the rest? Honestly, no.