For me it was my first Hard drive for my A600..it was a damaged Hitachi 345Mb laptop superslim HD which was extrememly expensive in those days but I got it free cuz the guy said it didnt work for his PC. But I knew that meant that it didnt necessarily mean it didnt work on my Amiga.
Boy that practically changed my life!
Second place would definitely have to be The Full Motion Video Card for the CD32. What a feeling that was when I showed my PC owning friends the peices of crap they owned, when even a 2 megabyte Commodore console blew their Pointless Contraptions (PC) to smitherines! I did it with one of the Highest quality VideoCD's ever made........FORREST GUMP! super excellent compression made by PHILIPS.
Third is of course My SX32 Freakin Pro....what a system.
and forth I think its gonna be when I add USB to my A600
Fifth will be when I add a DVD RW internally to my A600 or CD32