I'm pretty much using a whole bunch of calculators.
My most precious ones are the HP 32S II and my Philips SBC 1745.
I got the Philips one from my brother after he finished school, and it took me through school as well.
I rescued the HP from the office dumpster. My colleague got it because he requested a new calculator. The department assistant ordered it, not knowing anything about calculators. I assume she just picked the most expensive one at the time. The guy threw it out because he couldn't deal with the RPN.
But those two are mostly for memories.
For actual work I use my TI-92 II at home, and my TI-89 Platinum in the office.
The 92 helped me a lot while studying, and a while ago I bought the 89 because of its portability (and because it doesn't scare my colleagues as much
)
I really like the graphing abilities and the representation of formulae on the rather big display.
Also being able to program and use apps like TISpice is a nice treat.
If I'd be set to zero and had to chose, I would buy the TI-89 again.
For me, it's the most versatile of all calculators I have tried.
btw: To give it a try, I picked up the HP SmartCalc 300s a while ago. (yes the blue plasticky one)
While it's a decent and cheap (10 EUR) every-day office calculator, its build quality certainly does not deserve the HP logo.
It's a bid sad, that HP missed the opportunity to produce something great again.