I bought my Casio FX-115 Solar Calculator 25-years ago, probably in Malaysia. It replaced my then aging Texas Instruments calculator. The thing I like most about it is that it doesn’t need batteries. None. You can pull it out of a drawer after a year and it just works. No fretting about whether you left it on or not, and I’ve never needed to replace anything.
The new ones come with a dual solar and battery combo called “solar plus”. Don’t be fooled. The closest new equivalent is probably the Casio FX-260 Solar ($9.99), but that model doesn’t have some of the features of the FX-115.
As far as calculations go, it has pretty much all anyone would need. It has a nice friendly EXP button for scientific notation and infinite levels (18) of parenthesis. It converts and computes in alternate number bases (binary, octal and hex) and does linear regression.
The plastic is a bit scuffed after riding around in my backpack all these years, but it’s been wet and recovered. It also gets really sluggish when used outside in sub zero (C) weather.
[Note: The FX115 comes in two very similar models, the ES and the MS, that feature minor differences. However, it appears that the MS is preferred by some math teachers, and is approved to be used on many licensing exams (where as the ES has in the past not been approved for some engineering exams in California.]