What is your preferred calculator programming language (for any TI graphing calculator - this isn't necessarily which one you think is the best, just which one you usually prefer to program in)? I personally like C for the TI-89/TI-92+/Voyage 200 the most. It gives me pretty much the speed and power of assembly (if I need any additional speed or power - which I never have, so far, and it would more likely be power if anything - I can always just combine assembly files into my projects or use inline assembly), yet it is much easier to use and less error prone. There are also some really awesome libraries made for it, such as FAT and ExtGraph.
I also like programming in TI-BASIC for the TI-83+ series and Z80 assembly for the TI-83+ series and the TI-86, but not as much as C for the 68K series.
I havent really done much with the ti 89, since I dont own one.
I'd have to say assembly for the Z80 calcs. It really puts you in touch with the raw power (haha) of the zilog processor.
The even cooler thing is, that I found a pretty good on-calc text->assembly compiler, it is pretty slow because of the basic interface, but it gets the job done, as well as having a good on-the-run error correction feature.
I'm not an advanced programmer, but I've always programmed a little bit in TI-BASIC. I learned how to do some cool stuff, but i never really developed the skills through projects, but i can always make a quick math program if i need the calc to do something i don't feel like thinking about when I know a test is coming up.
The programming language i prefer definitely has to be the TI-89 BASIC. This is, of course, because it's the only one I know how to use with any amount of Skill.
89 BASIC is also a lot of fun. I've recently started working with C again, and I forgot damn near everything. The first thing i did to reteach mysef was develop a simple selection sort, program it in 89 BASIC, and then port it to C. IT helped me relearn the code much faster.