HP32SII got me through physics & maths II exams without all the tedious memorisation. It looked innocuous enough - certainly not programmable to the extent it actually was... a godsend for high-school.
Upgraded to an HP 48GX sophomore year of high school. It worked well for math and physics coursework, AP Calculus BC, and the SAT-I math section. The IR serial port's LED was so powerful, there was a learning TV remote app that could control TVs from ~60-100' (20-30m) away. The ability to beam software to other calculators was just shy of the invention of the app store.