The greatest HP breakthrough Scientific calcs

[Kostas Kritsilas]

What springs to mind immediately is the HP-35, of course, the first scientific calculator. no, it wasn't programmable, but it was the first scientific calculator that impacted a wide range of fields, from science to engineering, and pretty much obsoleted the slide rule. It was not programmable, but you asked about scientific calculators, not programmable ones.

Second one is the predecessor of the HP-67, the HP-65. It pretty much paved the path to the HP-67, and arguably the SR-52 and TI-59. First calculator with magnetic card storage. I am not saying it was the best of the HP line, but it was the first, and one of the first programmable calculators.

I think the HP-15C fits into this as well. In many ways, it is more advanced than the original HP-41, with built in functions for matrix operating and operating with imaginary numbers, as well as a workable solver. It also had a lot of programming steps for a relatively low priced machine.