HP calculators were an important part of my formative years. I have a 12C, 15C, and 16C (all original models). I also have an HP-35 (red LED digits) within reach right now. That was the calculator I used for high school exams, because we weren't allowed "programmable" calculators so I had to go a bit retro for the time.

The 16C was an interesting model. It had a lot of potential capability with the different word sizes and bitwise operations, but I think it fell short in practice because the operations it could do just weren't that useful.

My favourite model is the 15C, it got me through four years of math, physics, and computer science university classes. The integration and matrix functions were super useful because it was hard to do some of that stuff in your head.