My calculator
Decimals, with Engineering Notation (Wikipedia).
Learn how to use an RPN calculator while you're at it.
I highly recommend the HP 35s. This calculator is HP's non-graphing RPN calculator, and is approved for the FE exam.
RPN takes a little getting used to, but once you use it for a few days, you will find it to be both faster and more accurate than non rpn calculators. It does everything the TI 36 does, it just does it faster.