HP15C is amazing but so is the HP42S[1] and HP35S, with the 42S in particular considered by some people to be the best scientific calculator ever made. I have an HP35S which is great. I also have a swiss micros DM42[2] which is an absolutely fantastic calculator based on the Free42[3] opensource emulator. It's much more powerful than the HP42S while remaining 100% compatible[4]. The only downside is the keys are quite a lot stiffer than the HP42S.
For finance specifically the HP12C is the mac daddy. I have two of them and they're completely fantastic. They took me through an 8 year finance career and even though they are 20 years old now they are both still on their original battery.
[4] The processor is much much more powerful and it has 1000s of times the memory and essentially unlimited stack space. The instructionset is slightly expanded in a few useful and thoughtful ways. And it has a USB connection so you can upload and download. But original HP42 programs run on it unaltered.
For finance specifically the HP12C is the mac daddy. I have two of them and they're completely fantastic. They took me through an 8 year finance career and even though they are 20 years old now they are both still on their original battery.
[1] https://en.wikipedia.org/wiki/HP-42S
[2] https://www.swissmicros.com/product/dm42
[3] https://thomasokken.com/free42/
[4] The processor is much much more powerful and it has 1000s of times the memory and essentially unlimited stack space. The instructionset is slightly expanded in a few useful and thoughtful ways. And it has a USB connection so you can upload and download. But original HP42 programs run on it unaltered.