What was your most fun and rewarding upgrade?

Measuring impact per dollar:
Springs under (heavy) turntable.
Replace rack with concrete  blocks
$10 interconnects that bested a $275 set.
Replaced turntable bearing
Replaced stock tubes
PTFE tape on glass fuses

It would have to be my most recent upgrade to my new Bryston 4B3 amplifier. A new friend came over today and happened to hear music playing and asked to see my system. I had no idea he was a musician and had a few albums on Qobuz. So I put him in the chair and played his own album and a few others. He freaked. So he gets home and wants the list of every component after asking a bunch of questions. His Uncle is an Audiophile and wanted to know more as well. His quote from his text " I can’t handle my stereo now, So Bad, And its better than 98% of households. Yours is like other worldly bro. My Uncle want’s to sell his car now and go shopping. 12k? That’s encouraging, tho that’s a fortune. I thought it was a 30k setup based on the sound. I can’t imagine it better than that. "
Yes, Rewarding to hear confirmation from someone else who spends a lot of time in a studio and knows music. He had never experienced Maggies properly driven in a treated room. I have no other friends interested in audio so this was a first. He heard stuff on his own album he had not heard before, including his breathing. (Acoustic with no vocals). Needless to say I am pleased because I have never been able to get an educated opinion and to me it is normal everyday music.

My Mytek Brooklyn Bridge. A universe of music at my fingertips. In the best fidelity I've encountered in my sixty-plus years of listening, whether the source is vinyl, CD or the internet. Could I do better than the BB? Probably. But at the moment I just don't care.

My current speakers I got about 2.5 years ago. I'd really struggled finding speakers I loved flipped through a bunch then we bought our small condo I traded for a pair of original JMlabs Micro-Utopias. I liked and ran them for almost 10 years,  then I went to my first audio show. I realized something was missing,  a more dense fleshed out midrange.

I traded those speakers, and cash, for my current Audio Note J-lx's and could not be happier, This is the sound I've always wanted the 8" driver really gives me a great midrange and plenty of bass for my small room. By far my best upgrade and I've had some good ones in the past.

Room treatment seems to be winning the general consensus.