HP 35s Scientific Calculator
The HP's main claim to fame is reverse Polish notation support. Some people may find that perplexing, but it allows you to enter long expressions without using parentheses. Expressions are entered from the center outwards. The battery-powered HP features over 100 functions and 800 storage registers. It supports fractions and has a two-line, 14-character display. Forty-two built-in physical constants are included. It supports standard algebraic entry as well. It is quite pricey but at the time of writing there is a promotion where you can at least get a free HP 10S calculator in a bundle with this one. This should get you through any class at high school or college not requiring graphing.