Biggest "bang for the buck" audio gear you have owned(in your history)?

Thorens TD-160 paired with a Stanton 681EEE cartridge. I paid around $300 total in the mid-70s, best money ever spent for ROI.

After a number of years fiddling with audio I settled on an AR turntable with a Shure v-15 type iii, a Dynaco PAS 3x (bypassed tone controls), a kit-built ST-70 with Mullards, and FMI stand mount speakers (using their own heavy gauge wire). That modestly priced system woke me up!  I had by far the best sounding system in the whole dorm!
Forty years later, I'm blown away daily by a pair of Harbeth P3esr speakers driven by Odyssey electronics and a Hana SL mounted on a Technics SL-1200GR. For my listening habits and room this system is amazing.
Some may blush at the thought of a $10k system being as good as this. I don't obsess about my system at all and just listen to the music.

I downsized my $100k+ all Shindo rig to a pair of Kef LS50 Wireless II with matching KC-62 Sub. I'm not about to say it's better, (it's not) But running Roon directly into them it does blow the doors off most other systems I've heard at similar to double the price. None of which is important, what is important is that I now listen to it daily vs. having to find time to fire up and sit down to listen to the "big rig". Now I can sit and listen, have it in the background, crank it so I can hear it out on the deck, impress the neighbors etc...

Equitech 2RQMade my whole system sound like it was twice as expensive.That, or iFi Zen Blue bluetooth DAC. $129 streaming front end.$129 source connected to $18,000 worth of other components.Sounded pretty dang fine! Lumin U1 mini was big upgrade but cost 16 times as much. Doesn't sound 16 times better. Haha.

Rockwool. About $900. 
Made at least a 30% improvement to sound that I thought was already very good.