My favorite calculator (I have two of them) is the HP 35s...

The major advantage for me is that it is programmable. I can keep it loaded with oft-used computations as well as quickly constructing on-the-run calculations that will be repeated for a particular job. It can operate in algebraic or RPN (Reverse Polish Notation) mode. I use the latter exclusively because of many years experience with assembly language but the former is there for folks who don't get it. Runs forever on two 3 volt coin batteries.

I also have, but don't really use, a full-scientific calculator I bought for 99 cents at the local dollar store. I bought it to prove to a colleague just how cheaply calculators can be had.
Thanks Marv...that is a pretty fine calculator for the price. Not sure how I feel about the Assy Language though...been a long time and didn't do it day in/out.

I dropped and killed my TI85 a few years back and couldn't justify another to replace it as I don't do that stuff much any more or need a graphing one. However I may now that I did some research on 15C's...Wow I could sell mine (my first awesome calculator circa 86'). Pulled it out of the drawer and it still fired up! Ebay for them is ~$150-$400 and the new "Limited edition" is . I got mine from a friend who worked at HP for cost!! I kept all kinds of formulas in that thing in the day. Still have a 12C too. It's amazing how far they have come from simple math with Nixi tubes (TI??) to doing everything but wipe your brow and serve coffee.

Thanks to you and Frank S for the details & qualities of forming taps and dies!! ~PJ