What was your most fun and rewarding upgrade?

Biggest effect for me was replacing a Magnavox console with a Fisher three piece system.  -- My first step into the exotic world of audio components. lol

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.