What one change to your room, system, or setup made the biggest improvement?

For me it was adding a distributed bass array (swarm). Solved most of the major problems with the low frequency response quality of my room. Now all I need to do is "tweak".

I have been lucky and always owned since the 1970s reasonably priced quality turntables, arms, cartridges, amps and speakers. Each change incrementally improved, but the TWO most standout moments l most appreciated came more recently.

1, Buying my first sub, a Rel Quake over twenty years ago. It was entry level but what a resounding success story. There were no doubts with instantaneous approval.
2, Upgrading about six years ago from twin Quakes to a pair of Rel T9i’s. Again a massive leap forward, and presently leaving no feelings of a need to improve on them.

Room treatment (DIY), placement of speakers. --- Pearlacoutics Sibelius speakers, getting REL T9 dialed in right. --- Zesto SUT w/ Rega Naia

Added Kimber kable 8tc speaker wire  Made a huge difference.

After I bought my Harbeth speakers I made some diy bass traps and put them on the wall behind the speakers. Music just jumped out at you. That's the best description I can think of. I did a before and after with my son one day. He heard it too. It wasn't subtle.