My HP-48GX calculator.
i cant even remember how much this thing cost new. $199? i got it back in 1992 and i still use it daily. i do remember worrying about how much it cost over the TI-85.
the screen is now starting to fail, but it still works. i use it daily.
i found one on a big highway job across the desert, and bought one for $50 on some toyota tacoma forum..so i have two spares. i put my programs on all of them. i have enough calculator to ride out my career.
i cant believe the value i got out of this thing.
what is your single best purchase?