Casio FX 992 always seemed very ergonomic and easy to use, although it might be considered a bit vintage now. It had a sheet of physical constants, so for example 90->const enters the speed of the universe in m/s. Having a random number generator built in can be quite convenient sometimes.

I have a whole desk full of calculators. The two I use most are:

Sharp EL-W516X A very capable little dot-matrix device, with some limited programming / macro ability. Performs all of the calculus, stats, matrix, and combinatorics functions I use regularly. This was my go-to calculator until I started using the…

Casio fx-115ES PLUS At some point, I had to write some exams which only allowed a few calculators off an approved list. This was the one I picked from the list. It is a very competent little device which can do anything the Sharp unit above can. After getting over the novelty of having to learn a new keypad, I found it nearly identical to the Sharp in usage.

I think that both calculators are effectively identical in terms of function, but the Sharp may have a slight advantage in terms of its interface. I would wager it takes slightly fewer keystrokes for the same operation on average as compared to the Casio. I also didn’t really take advantage of the programmable aspect, instead using the four keys for the common multipliers I use in my work: kilo, milli, micro, nano. I liked how the display would read in “natural” terms like “2 * pi * 10k * 100μ” instead of having a tonne of “10^n” terms in the line.

The Casio has the advantage that it is typically on just about any approved calculator list; if you’re taking exams down the line, consider getting used to an approved calculator now so you’re not wasting time searching for buttons later. It’s also the more popular choice, I think. I saw plenty of students, TAs, and instructors using this calculator which could make it easier to give and receive help on it.