What's the best and/or favorite piece of equipment you've owned? And do you still own it?

B&K 202+ power amp.Still have it but it needs repair.

My Aiwa AD F-770 three head cassette deck. 38 years and still going strong with nothing more than a belt change. Also, my Marantz st6000 tuner. These are the only things remaining from past systems.

IMO, once you achieved a very high level of synergy (including your ears/brain) it's VERY difficult, maybe impossible, to pick one single item in one's setup. 
How tricky is it, to chose between Burmester speakers, Levinson CD, DAC, preamp and PassLabs power amp?!? 🥴 
Am I the only one here with such a difficulty in answering the OP's question? 
M. 🇿🇦 

My favorite piece right now has to be a Lyra Kleos cartridge. I upgraded to a VPI Super Prime Scout but installed my old Ortofon cartridge. When I installed the Kleos, it didnt open the door to high definition sound it Kicked the door in with both feet. Couldn't be happier.

My all time favourite piece of kit is my Michell Gyrodec bought in 1992 (for £659 !!!) and subsequently upgraded to Orbe status. Now with SME V tonearm and Koetsu Black cartridge and still in regular use to-day. I have had other turntables down the years and wish I had had the funds to keep them rather than having to trade them in !! (Pioneer PL12D, Thorens 160 Super, and Dunlop Systemdek III, the latter equipped with an outboard power supply which made all the difference).