Hewlett-Packard, in my opinion, made the best ones. I still love Reverse Polish Notation (RPN). Texas Instruments made many different types but their quality was never as good as Hewlett-Packard. Sharp and Casio also produced a nice line of interesting calculators.
The internet provides many online calculators. There are also spreadsheets and of course Mathematica. Some interesting online calculators include:
What is your favorite calculator?