THE BEST: By far, the best calculator of all these is the simplest... The TI-30Xa. It is also the cheapest ($9 at Target). It is the easiest to use and is a workhorse of a little calculator. Ready for anything you throw at it. It is what I use. And added plus is that it is cheap enough to not make you cry when you lose it. Just go buy another one. And it is typically one of the stock items for places like Target, Walmart, Staples, Best Buy, etc...
Sadly, when schools start up (August/September) is when many stores (and Amazon) are out of stock on these. Shop around, you can find one in person in a brick and mortar store.
You're going to pay more for the TI-30X IIS coming in at $18 - although often on SALE for around $12. It has 2-lines on the display and is therefore "fancy". Something I dont like... no single punch EE key for scientific notation. Sure, you can (and should) hit 2nd and then EE but that is more punching. You can edit your numbers that show - so maybe thats a "win" for you on this one. Oh yeah, seems to be the one with the most color choices. So if pink is your thing, go for the TI-30X IIS.
And finally, the fanciest of all the SAT and ACT (and chemistry) approved calculators, the TI-36 Pro. It's going to set you back coming in at around $24 - although often on SALE for around $18. It has 2-lines on the display and is also "fancy". It does have the much used single punch EE key for scientific notation. However, the learning curve on this thing is steep. So many buttons and functions. It makes my head hurt. Just say no, and go for one of the others. But hey, if you really can't help yourself, we will let you use it on exams.
We will not let you use graphing calculators or any type of programmable calculator. This means all the fancy TI calculators in the 80-series are out (and definitely not the Nspire). Do not bring this type of calculator to the exam. We will see it and tell you you cannot use it for the exam and then you will be doing all the math by hand. You Will also not be allowed to use your phone or any other electronic device that isn't a basic scientific calculator.
What math are we usually doing? Add, substract, multiply, and divide. That is 90% of what we do. But sometimes the numbers get really big and that is why you need a calculator with scientific notation on it. The other 10%? Depending on the class, we will take some lograrithms (both natural and base-10). We will do powers like xy and also roots. That's about it. No differential equations and integrals. No plotting... at least not on our calculators.
Speaking of plotting... if you're going to do plots, switch to a computer and use Excel or some other program for that sort of thing.
And finally... that trusty TI-30Xa will be a great little calculator for all sorts of little calculations you might need to make in the real world. Great looking display and easy to read. So good.
Other brands are "fine" but make sure they are just plain scientific calculators and not programmable ones. HP, Sharp, and Casio have them and those are OK even though I find them clunky (or worse) to use. Then there are the off brands with brand names I don't even recognize. I'd avoid them - but, if they meet the low-end requirements, then ok.