HP 48GX by Hewlett Packard. I picked this up at a local flea market for 25$. It goes on eBay for upwards of 100 bucks! I love the HP's reverse polish notation. If you haven't given an HP a try, do! Next up, HP 50 GX.
The HP-15C Calculator Scientific Calculator was an RPN (Reverse Polish Notation) calculator that was popular with engineers back in the '80s, and it was manufactured from 1981 through 1989. It was a programmable calculator that had built-in support for complex numbers, matrix math, numeric integration, and root solving. It's small size (5” x 3.15”) allowed it to be carried in a shirt pocket.