Personally, I recommend the TI-86. But I am biased because I am very familiar having used a TI-85 since junior high school (I don't actually have an 86, my brother does, however).
Perhaps a little background would help:
For quite a while the two major TI graphing calculators were the TI-82 and TI-85 (the TI-80 and 81 were around as well but are less powerful than the 82 and 85). The 85 is a bit more advanced (allowing such things as variables with names longer than one letter), and has a slightly different interface. Then came the TI-92, the big powerful calculator. Because its keyboard has a normal QWERTY (computer/typewriter) style layout it is not allowed to be used on some standardized tests such as the SATs.
A couple of years ago, right about the same time the AP Statistics and Probably test was introduced, TI introduced the TI-83 and then the TI-86. Essentially, these are versions of the 82 and 85, but with additional functions geared towards the AP Stat test.
Finally, the TI-89 was introduced, essentially a TI-92 but with the traditional TI graphing calculator layout. I guess TI realized they'd sell more 92s if students could use them on the SAT.
Becaue of my own experience with the TI-85, I reccomend the TI-86. It also seems like its designed for a bit more advanced college mathematics. However, if you don't have any experience with any, you might want to get whatever your school recommends, probably the TI-83 because your teacher will be most familiar with it and thus will be able to provide the most assistance.
Re HP's: With them, you have to enter data in reverse polish notation (for example 2
3 as opposed to 23 on a TI). This can take some getting used to, but I have friends with HP's and they said it's not too bad. Also the infrared capablities are a concern on standardized tests, often though if you just put making tape over the end of the calculator that blocks it and you'll be ok. Again, check with your school.