One other factor that others haven't yet covered is that the different lines had different capabilities, e.g. the T-89 had Computer Algebra System symbolic manipulation meaning it could pretty much solve many types of math problems on its own, so it wasn't generally allowed in school. And then the Ti-85/86 was a step down, but had matrix support that the lower models lacked, so it was necessary for some specific types of classes.

My favorite was always the TI-85/86 line. I loved those F1-F5 buttons right beneath the screen, which made the interface overall better to navigate. The first programming I ever did was on one of those (either the 85 or 82, can't exactly remember at this point which I owned first). And, the only thing of note I ever had stolen from me was a TI-82, taken out of my unattended backpack by another student during gym class :( (And I had even carved my name into the back of it with a knife, so it would've been identifiable.)