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

Absolutely, and without a doubt, two pieces come to mind for me. 
- Rogue Cronus Magnum II Integrated Amp
- Audioquest Niagara 1000

Couldn't live without them.

The biggest improvement I made, and not for killer dollars, was when I bought my first pair of serious speakers--Magneplanar MG -I.

1. Vintage tubes. For around $100 each I’m not sure there is a better bang for the buck out there.

2. Empirical Audio Off-Ramp 5.  This USB converter absolutely transformed my digital system for less than $1k used.

3. Audience aR2p power conditioner for $695 new. I’ve since moved on to the TSOX version at considerably more money but the incremental improvement really didn’t justify the cost. The entry level version is a tremendous value.

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.