tl;dr: RPN - ride or die. ;)

Sometimes, for vintage reminiscing, a real HP48GX.* It got me through AP Calculus BC, SAT-I, and EE/CS. And doubled as a learning TV remote & TV-B-Gone. I also have a virtual HP48 on my phone, tablets, and computers.

In middle school, I had an HP 32SII. I remember being able to hear** either the processor clock or data line(s) of each of the 32 and 48 because one or more components acted as inadvertent audio oscillators that would make noises during computation (and during idle on the 48 because it had to keep redrawing the clock).

I've used R, MATLAB, pspice, and Mathematica in the past.

I'm a fan of Xcas and Wolfram Alpha now.

* I also have a collection of HP 48[GS]X cards, the printer, and the overhead projector adapter. In lieu of the overpriced official HP 48G to computer RS-232 interface cable, one can be easily made from a Sony CD-ROM audio cable.

** With my much younger ear pressed up to it.