Usually Windows calculator. My keyboard has a dedicated Calculator button that I've gotten so used to I won't want a keyboard without it. For anything complex: Excel, Mathematica/Wolfram Alpha, custom program.
For college I used a TI-30XS. Multi-line helps a lot. Only used it in physics, chem, stats. Core mathematics courses had a complete ban on calculators at my school.
I had a TI-86 for highschool. I loved that thing, it had the best menu layout of the 83/85/89, etc. Unfortunately I let my dipshit nephew borrow it for his HS classes and he gave me back a TI-83 lame ass edition. I think that idiot traded for a pack of smokes. I told him to just keep the 83, partly because it was a POS, but mostly because graphing calculators are of very limited use. If it weren't for highschools strict requirements they wouldn't make 90% of their current sales.