Hah, I am a big fan of RPN and dc calculator. I used to use bc for quite a bit of time, but for some reason I moved to dc and never looked back. Doing calc on stack based calc is so much easier :)

I used my HP16c up until about 2014. its ability to do hex/bin/octal was its selling point for me. Alas the stickiness of iPhone to be in my hand more often then not it finally drove a stake through the heart of usage habbit.