I think that very few people dispute that the HP42s is the top RPN calculator HP ever made. This definition discards all RPL calculators, that could be a worthy competitor for the title. However, except the Clamshell scientific-graphic calculators (the diverse HP28 variants), the RPL machines were big and difficult to be considered pocket calculators. The only other possible competitors were the HP15c (design factor) and HP41c (expandability). But on raw power, the HP42s handily beats both of them.
Once HP had moved to the Saturn architecture, and the HP41c started to show its age, HP decided to create a machine that was software-compatible with the HP41c - and much more. On the other hand, compatibility with all the moldings made for the Pioneer series forced the HP42s to be born without expansion ports - it only had a infrared port to communicate with the HP82240 printer.
To make it compatible with the HP41c, HP went back several notches in some of the attributes of the HP42s. One that drives me nuts is the solver. It still uses the RPN solver, equivalent to that of the advantage pack: you create a program and then you apply the solve routine. It had several advantages over the original: by defining the soft keys at the beginning of the program, they could be made to appear in the second line of the screen. But on the other hand, the solver that the HP17bii sibling already had was much more convenient - and I am still using it, 20 years later.
Other feature that keeps it midway modern and old design is that it has both registers (like the old HP41c) and variables (like the new RPL machines). It also has true matrixes and complex numbers built in - as it should be done. Complex numbers occupied only one stack level - like the HP15c but different from the HP41c additional modules. Complex numbers appeared naturally when the situation called for it (ex.: the radix of a negative number). Matrixes could be properly edited on the screen.
The machine had a menu system well organized, that let you arrive to your function in very few strokes. You could call it anyway with the XEQ button just like the HP41c.
I have one for sale. I have so much respect for it that I never use it to avoid damage, so what’s the point of keeping it? It has the particularity that its main memory has been increased to 32kb, instead of the original 8 kb. I have not performed the surgery, but you can read about the particulars here:
Not that I have ever used the 8k original…but it always feels better to have a memory reserve!