I still remember fondly my system with DeVore 093s powered by Cary Audio Rocket 88 amp and Cary SLP98P preamp, Thorens 125 MK2/SME 3009, various cartridges.
I shouldn’t have sold any of it. I should have bought a bigger house instead.
I miss my Decware SE84UFO3 Monoblocks with all the Anniversary mods. I don’t regret selling them because that allowed me to afford the Decware SEWE300B which I have now and adore. . . but I wish I had a place for them here in the home as they were just different enough with a certain inner detail I sometimes miss.
I bought an Audio Research SP6B preamp back in the early 80s. It was my first piece of tube gear. It never impressed me. I sold it a couple of years later. I’m curious now how it really sounded. I’m sure the crappy interconnects and associated gear that I had in the chain at the time never allowed me to really hear it.
Common occurrence for sure.
It was special to me at the time and offered incredible connection flexibility:
I bundled it with some other gear to help with the purchase of some much better speakers (which I still own today).
My components are no doubt “better” today, but I wish I still had this to play with. It was in immaculate condition and performed well - but that was a long, long time ago.
Cheers.
The only thing I miss after 50 years in the hobby is a Bedini 25/25 Class A amplifier. Full bipolar goodness Class A, it was a gorgeous sounding amp. A bit of a toaster, though! 
The casework made it look like it was built by moonshiners in a mountain shack, but I loved that amp. Probably couldn’t stand up to something like a Pass XA25 of today, but at the time it was tough to beat with the appropriate speakers. As a young professional with a new (and at the time expensive!) house, I had no choice but to sell it to move up the food chain. Sigh.
BAT VK150 has the sweetest midrange than any of my amps, but I do not miss the heat and retube cost. Actually I do not miss it as much now as before after my new amp arrived. 
This is really not bad at all considering how many gears have passed through my system in the last few years.
I probably have a “childhood memory” of the system I had while in the military (mid-70s) but it sounded delightful to me at the time. I know the spirit of the thread was to name a single item but I miss them all
- SoundCraftsman Pre/Equalizer (same one in @scotte1 's post earlier)
- Harmon Kardon Citation 16 Power amp. LEDs were the latest/cool tech then!
- Nakamichi 1000 cassette deck
- Pioneer HPM-150 speakers (crazy UFO-looking Omni-directional super tweeter)
- Harmon Kardon Rabco ST-7 Linear Tracking Turntable
- dBX 3bx 3-band dynamic range enhancer (3 rows of LED bliss!)
Amazingly, the “speaker cables” were glorified zip cord - I don’t know how we managed
I still have the Rabco but haven’t used it in many years. The linear tracking feature was a mechanical nightmare.
Mine was in silver
I wish I hadn’t traded in my Sprout that got me started on the PS Audio adventure. It was such a bargain. Looking forward to what Scott is able to concoct.
My pre-tinnitus hearing, sigh!
Not sure if this will count but back in high school, I spent a lot of time in my friend’s basement hanging out and shooting pool. We always had music on and his dad had a set of big Ohm Walsh omnidirectional speakers. I’m can’t remember the electronics, but we had a lot of great times down there. I’ve actually thought about reaching out to my old friend and telling him his dad should call me whenever he wants to sell them. I don’t know if it was the age I was at, the first nice system I had really listened too, or if the speakers were great at all, but we really were enjoying music back then.
It does tempt me to hear some MBL speakers at some point. I’ve never had the chance to hear anything else like that.
Funny this topic shows up now, because lately I’ve really been missing my Klipsch Heresys. So much so that I’ve been browsing US Audio Mart and Audiogon looking for another pair.
There was just something about that big, larger-than-life, forward presentation they did so well. Every now and then, I really miss that live, in-your-face energy and immediacy that only speakers like the Klipsch heritage seem to deliver.”
Sumiko Talisman S phono cartridge.
I cringed when I shattered the sapphire cantilever. I wish I had it rebuilt.
https://www.stereophile.com/content/talisman-s-mc-phono-cartridge