I believe that I have owned a total of 4 HP calculators, but the one that I like the best is the HP15C. Earlier calculators had LED displays which used a lot of energy, requiring a large rechargeable battery pack. The 15C has an LCD display, and is powered for years by 3 tiny watch cells.
At the top in this picture is my old 15C, acquired about 1983. About 10 years ago the display suddenly developed some dark patches at the edges, but this has not yet affected calculator function. Below is the new “limited edition” 15C, temporarily available. I couldn’t resist, given the prospect of my old calculator some day failing.
I also have a later model calculator, the HP48. It has a 4 line display and more fuctionality than the 15C, but it is larger and heavier, and I do not really use the extra capability. Also, it operates slightly differently in a few cases. For example, when I want to square 7 on the 15C, the keystrokes are: 7, enter, X. The 48 takes one additional keystroke: 7, enter, enter, X. I think that this change is the consequence of having a 4 line display. But I find the difference to be annoying.