Mine: HP 15c. Minimalistic design, intelligent use of all keys (triple and 4 uses depending on the context), dependability, long battery, capabilities, pocketability
I prefer HP 42s. It is far more powerful than the 15c. It is faster. The matrix operations are better. It is still the best RPN calculator ever.
José L. López
The most useful in my daily work is the 12c. A timeless classic. Very nice, with an unusual format. Stresses on the desk. Easy to use, simple and robust. Fits well in the pocket and on the suitcase.
For me the best of the HPs is the 48 series for multitude of reasons. I use the 42s for everyday calcs (and that’s because my 41cx kaput, cry, cry), but the programming language in the 48 series and subsequently the 50g (which is a 48 on steroids) has no equal among calculators. My 42s is near its demise, so I think I'll do one of those WP 34s conversions. I downloaded the software and so, far like it. My 48sx is lovable, but too bulky and the manufacturing of the 50g is crappy, but I still think the RPL and System RPLs are genius.
I'm really enjoying the WP-34S. The small form factor means that you can operate it reasonably well with one hand - a feature I really miss from my old 29C. It has scientific functions, computer science functions of the 16C, alpha functions of the 41C, matrix functions like the 15C, complex number support, lots of statisitcs, and the display is pretty good.
i still love and use my HP 48GX, illo tempore, expanded with a 512KB(128x4) and a 1MB cards from Klotz.
Although it's been some time this thread started, I'd like to add my all time no. 1: The HP-41 series. No other calculator ever offered such a varity of peripherals and with this versatility. Due to technical advances, my no. 2 is the 48/49/50 familiy. Aworthy successor as no. 1 should be a combination of the 41's expandability with the 50's large screen (of course with full colour and a much higher resolution), CAS, RPL and function catalogue.