United States
Texas Instruments
The calculator most US math teachers teach on. Older models keep getting OS updates, and the used market is deep enough that almost any model is easy to find.
When a teacher walks the class through a keystroke sequence on the overhead, it is almost always a TI-84. That habit is a big part of what a TI is worth. Some of that value is the hardware, and a lot of it is matching the calculator on the board.
Products scored 4
Tech
TI-84 Plus CE
The calculator most teachers demonstrate on, with a color screen and a battery that charges over USB.
8.4
Tech
TI-Nspire CX II
Strongest non-CAS graphing calculator here, approved on the ACT. Make sure the listing isn't the CAS model.
8.2
Tech
TI-84 Plus
The monochrome original that schools bought by the crate. Indestructible, everywhere on eBay, and visibly old.
7.7
Tech
TI-89 Titanium
A computer algebra system in a 2004 body. Allowed on the SAT and AP Calculus, banned on the ACT.
7.2