First, it sure is nice that we don't actually have to choose just one slide rule and stick with that, but if you had to choose just one, which one would it be?
I think a lot of folks would choose "the big slide rule", like a Pickett Model 4, a Faber-Castell Novo Duplex 2/83N, a Nestler Multimath, a K&E Deci-Lon, or any manufacturer's "flagship" slide rule. Because anything you're likely to need to do with a slide rule, you can do with one of these big rules.
However, what if your use case is that you're not an engineer? I like my N4-ES, but I just never have the need for hyperbolic trig scales.
What, besides the number of scales, makes a slide rule the one rule that you would select?
I am partial to pocket rules, but I also like a rule that is easy to read. These two interests can be mutually-exclusive.
I am not an engineer, so I usually use a slide rule for quick multiplication & division, or I'll use trig scales to figure the lengths of legs of a triangle when I'm building something. So I don't actually need Log-Log scales.
I like a rule that is durable enough that I'm not afraid to carry it around and actually use it.
Of the pocket rules I have, my Aristo 867U Darmstadt checks the "legibility" box the best, and has a nice set of scales, even including a couple of Log-Log scales should the need arise. I'm not sure about its durability as a rule to carry around every day. Its case isn't very robust, compared to Pickett "Saddle Leather" cases.
The next candidate is my Pickett 700. It is smaller, and has enough scales to be useful, even including folded scales! It falls short on legibility, cramming all of these scales on one side of a little Pickett pocket rule. On the other hand, it's kind of an unusual rule, being an "Aerial Photo Slide Rule", so it has the extra quality of "I just like it". And its durable leather case has a window with conversion factors displayed.
The rule I actually carry and use most often is my Pickett N200-T. Having fewer scales, it's easier to read than the 700. I also think its later-model cursor should be more durable than the 700's four-screw flat cursor. It has trig scales, CI and L scales, but no folded scales. Also, this rule has a leather case with a top flap, so the rule is very well protected.
If I could only have one, though, I think it would be the 700. It's hard to fight the "I just like it" quality.
I'm glad I don't have to pick just one.
What about you? What "one rule" would you pick?
Solid choice. Powerful, portable, and beautiful.