What was your most fun and rewarding upgrade?

The most rewarding was my last : A used SOTA Sapphire III TT, a new Jelco 850 MK II arm, and a Soundsmith Zephyr III cart. No doubt, the most satisfying upgrade, to date.

Prior or that, my Vandersteen 2CE Sigs. It will take a serious upgrade to replace them. Now, it’s my ‘bang for the buck’ winner.

Wasn’t wasteful, as I sold it for what I bought it for, but an original Adcom Pass designed 535 amp. Always wanted one back in the 80’s, but after getting it, didn’t ‘do a lot for me’ after a while. Replaced it with a B&K 125.2 amp, which for me, was a huge improvement sonically. That’s now gone, and replaced with an old Belles 400A, which I like a lot, and a worthy successor.

Moving from living room to basement as a dedicated listening room.  I can put speakers wherever they sound best, it’s isolated from rest of house, and cement floors much better for sound — especially bass — than suspended wood floors. 

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.