HP 12C. I've never used it, but HP calculators in general are worth the extra money because:
1) They're always extremely sturdily constructed. The buttons have a great 'feel' to them, and the plastics are a lot thicker and better-quality than any other calculator manufacturer's.
2) The RPN system! It takes a while to learn and may turn you off, but I swear to God once you've learned it you'll never go back! Once you become proficient, you can input your numbers and get a result 12X faster than other people using 'normal' calculators. You'll have your answers while everyone else is picking away.