My first 80 megabyte 2,5" Conner hard disk for use in my A1200. I paid a whopping 600 HFL (~275EUR) for it back then (94? 95?).
But oh....the difference it made. Combined with a 28MHz 030 I actually could start using Workbench :-).
The worst? KCS PowerPC XT emulator board I bought for my A500. All of my friends had PeeCee's and I wanted to be 'compatible' so badly. In the end the XT emulation was useless (this was in the days of 386's). What a waste of precious money that was :-/.