Most enduring audio purchase

Vandersteen 3A Signatures. 

Although, they are not currently active, but occasionally get used for one reason or another, my Polk 5 Jr’s I bought in 79-80...can’t remember exactly. I have none of the old equipment from that time except the Polks, and will probably have them til I die.

I get 'The Nervosa' from time to time, but whether it's just because speakers are a pain in the balls to sell, move & change or it's even harder to find the right speaker, I find it's the speakers that last longest, followed by amps.
It takes me a longtime to both research & find the *right* speakers and a very long time to commit to changing them.
My Vandersteen 2Ci's, Thiel 3.5's, Maggie 3.6's and B&W 802 Matrix's over have seen *dozens* of luscious and a few not so luscious preamps over the last 30ish years. I'm a preamp whore. Love a few different sounds. Love tubes, then I love leaving a nice, hot SS on 24/7 again- it's shameful.
I've probably had a half dozen preamp switches in the last three years. I'm in the middle of one right now.
Amps, again, less than a half dozen- that's another thing. When you get a good one, you just stick with it and dick around elsewhere. Same for DAC's. I hung on to my CAL Sigma 2, which was so 12AX7 tube rolling responsive, for a very long time. I still think about how much pleasing music I used to get through that beauty. I might even retro digi-fit a new one. That was a lovely DAC.

I bought a pair of Proac Studio 1s back in the mid-90s and have had no reason to part with them - amazing high end from my admittedly limited near-field monitor experience...

26+ years ago I searched daily for 3 months (Im a realtor) looking for the right house. My living room is 19 x 35 X 12ft peak with open beam ceilings
While not ideal for hi-end stereo (non-existent unless you build it, one of my friends did just that)  I could work with it, AND, I love open living spaces. TV system on the long way, audio on the 'short' wall