i have one SINGLE definitive answer. the answer has ever changed. the undisputed champ of "best money spent" my HP48GX - 26 years old maybe. i think i paid $150?! got me thru college, helped me understand calculus, got me past the EIT, got me past the PE-exam, used it to figure out my house loan BS, and it gets me thru the daily mundane math of civil engineering work. i still remember clearly, the first class instructions were to buy a Texas Instrument 85. pfffttt.. a few of us bought the HP, and the professors would tell us we were on our own. he would use the TI85. he walked the class thru.."now push this button...". we scrambled with the HP's and eventually got faster than the professors. bad ass calculators.
honorable mentions:
Casio Pathfinder Watch
toyota Tacoma.