I guess this is only tangentially video game related anyway, but it's really the thing in my "collection" or whatever that I would have the hardest time parting with.
So when I was a teenager (specifically starting in 1990), I got REALLY in to PC gaming. Like basically "fuck consoles" level. My best friend's dad was a game programmer at Sierra, he was super in to computer games, and he had a shitload of them, so that helped fuel my fire. I started off with a POS XT-based system that had dual floppies and a monochrome monitor, and slowly upgraded. First I got a color monitor, then I traded someone for a 20MB (yes, MB) hard drive. The next big move was to get a barebones-as-fuck 386 DX40. It was just the AT tower, mobo & CPU, a 3.5" disk drive, and a little bit of RAM. Was still using the 20MB HDD, and even moved one of the 5.25" floppy drives over even though the faceplate made it not fit quite right, because I saved a few bucks that way. Bought a 255MB hard drive at Costco to replace the 20MB deal. But the last piece of the puzzle was a sound card, because up until this time I was just rocking the PC speaker. But games were coming out by this time that could take advantage of something like a SoundBlaster card.
So my best friend and I both worked odd jobs all summer in 1993 to save money. He was mowing lawns and washing coffee pots for some neighbor of his who ran a coffee-machine leasing business. I was working at my step-dad's (although he wasn't my step dad yet) cabin up in Yosemite digging trenches for a new plumbing system. At the end of the summer, we both bought Gravis Ultrasound cards. These things were bad fucking ass. They were awesome for games like X-Wing because they could simultaneously emulate a SoundBlaster and a Roland MT-32 or Sound Canvas, meaning digitized speech and sound effects PLUS midi music. Some games supported it natively (like Epic Pinball and Blackthorne) and they sounded awesome, as well. I also spent tons of time fucking around with MIDI music and with MODs, which I would download off of BBSes.
So to make a long story kind of short, at some point several years later, my GUS died. Not sure how or why, but it just didn't work anymore, so I threw it away. But I have always kept a nicely-spec'ed DOS gaming machine around, and I wanted to get a GUS to put in it. I was looking for a long time because I wanted one CIB, and finally found one for sale in Germany (where the GUS was relatively popular). I had to sell my Vectrex collection to pay for it, but I ended up getting it. It's not like it's worth that much money, but to find another CIB Ultrasound would be very difficult, so that's why it's kind of the "holy grail" of my collection.