Which is the one to get? (I searched, but the posts I found relating to this were old, so I thought I would ask anew.)
In my opinion, the current all-around best graphing calculator is the TI-84. It should cover all math classes up to calculus, where you might need to upgrade to a more advanced calculator such as the TI-Nspire CX-CAS or the TI-89. It pretty much depends on the class and what level of math the calculator will be used for.
Going into Calculus-level math makes you competitive. If you’re going into Calculus, get a used TI-89.
If you aren’t interested in being competitive, then get an 84.
My kids who are both advanced in math and DS1 does competitive math both use a TI 83 or 84 plus. They both got one in middle school and are using it in high school. DS1 got a new one because his stopped working for programming. It is also allowed for all standardized tests. The TI89 and Inspire are not allowed on some tests. You can sometimes get good deals at Amazon.
My D’s school requires TI-Nspire CX CAS.
I would recommend the TI Nspire CX CAS. For the main reason that you can store some programs on it. The programs are allowed to be used for SAT math 2 and can help speed up some problems.
The Nspire has the same features as the earlier models but has the CAS.
If you’re planning to use it for competitions or for timed tests, you may want to check the trade-off between color and speed. I seem to recall that the black and white ones were a tad faster.
My S’s school also requires TI-Nspire CX CAS.
This calculator is allowed in College Board tests - SAT, SAT subject Math tests, AP Calculus tests - but not in ACT.
For ACT, S used his old calculator from middle school days, 83 or 84.
Another vote for the TI-Nspire CX CAS. Having said that, I would suggest asking the math chair first. Schools sometimes have specific requirements.
I’m just impatiently waiting for the schedule/teachers to be posted so I can contact the teacher and buy whatever is needed. Patients has never been my strong suit.
My kids have TI-84 plus silver edition- they needed it in middle school now use it for their Hill math classes and the PSAT.
DD in public school needed a TI-84. DS’s boarding school requires the TI-Nspire CX. He was told NOT to get the CAS model.
I used a lovely TI-84 until I got to Groton, where everyone purchases (or receives, in my case) the Nspire CAS CX whatever you call it. It’s very fancy. It also can’t be used on the ACT, and probably not SAT either, but if you guys say it works through calculus, that’s probably why everyone has it.
In exams/parts of exams that allow a calculator, the NSpire is allowed for SAT/AP exams, but @stargirl3 is correct that it is not allowed for ACT.
Schools let you know what’s required. Buy that model.
What have you done!!! I bought the CAS CX for my daughter and today she is telling me that she figured out how to install Nintendo emulator and play Pokemon on it, and claims that she deserves to play it for all the programming trouble she is going through.
BTW, the school recommends TI-89 but is totally cool with CAS CX.
No idea you could use CAS CX to play Pokemon! Not a problem with my D, but my son’s idea of dropping DD off at BS was playing Pokemon the entire 2000 miles! He told his sister that her school was a Pokemon haven! I guess I’ll be buying him the lesser calculator!
I think the popular one now is the newer Pokemon Go though. 
The CAS CX can emulate the old Nintendo with old Pokemon. Still fun.
My kid prefers the vintage Pokemon
I just had to buy another $100+ calculator for my daughter. Grrrr.