What was your most fun and rewarding upgrade?


I've got a decent setup:
PrimaLuna Evo 400 Integrated with KT120'S and Psvane 12au7s
Clearaudio Concept Wood with Hana EH Cartridge
Musical Surroundings Nova III with linear Power Supply
Ps Audio P10
Focal Aria 936
No upgraded power cables, but Big Sur Interconnects and $200 Wireworld speaker cables

What I'm curious about was what was your most rewarding upgrade? Also, if you were in my scenario, where do you think the most fun upgrade would be for you? What was your most wasteful or least interesting upgrade? What steps do you wish you would have made first and which would you have saved for last? Also, what upgrade did you love that maybe didn't give you the most bang for buck or was least valuable, but you enjoyed the most?

I'm considering a Herron Phonostage for my next experience. Or cables. Or isolation. Ahhhh the options (but the Herron phonostages...)
128x128j-wall
navships cables/Mundorf/Takman/Sparkoslab/Telefunken/Svetlana/F&T's all made  very happy high fidelity gains. 
Biggest bang for the buck, was room treatment.

Second was the electrical grid, maintain 120VAC, protect against surges (quickly) and high quality cabling and outlets.

The best improvement overall, was going to BI and Tri amping, with individual bass box control, The use of MB columns in a stereo setup, with my current monitors along with the control I have per enclosure and sections within that enclosure, have REALLY been the ULTIMATE TWEEK!!!

Things like, Sparco opAmps and regulators, valve rolling, and cables do fine tune them a step further, though.

Regards
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.