My best purchase hands down is my Guild B-301. Probably, for what I want out of a bass, my best purchase ever.
Probably my worst purchase was a combination of various things for a little project. Years ago someone had given me an old Stagg practice combo, 20W, I think, with an 8" speaker. It lived in the loft. I decided to carve it about, mount my Trace Elliot Elf in the top bit and put a better quality 8" driver inside. P. Audio one, 200W, cost me £50. I struggled to get the front board out cleanly and managed to snap it in half, the jagged bit tore through the grill cloth. So, rather than chuck it out the window I handed it over to a mate who did all the necessary stuff, put some nice Fender-esque front cloth on it, mended the front board, mounted the Elf inside with little brackets, fixed the new driver inside. Bloody thing sounds like a radio speaker mounted in a biscuit tin 🤣 I really should know better.