Best "Bang for the Buck" you have owned?

Superphon revelation preamp I traded an ice fishing tent for that I had in 1990 and now a Job 225 amp-  bought a HH Scott LK72 that was recapped etc in 2019 for $500.  Beautiful piece.

What a great question.  
I've been very lucky finding outstanding used equipment.  in no particular order -- and this spans the past 30 years:

Magnepan MG-1 speakers
Apt-Holman pre=amp
Technics SL-1200 turntable (the original; just purchased a new one for $1,000)
Sonographe SA-400 amp
Marantz UD-7007 BluRay player (used for SACD's)

What a great question.  
I've been very lucky finding outstanding used equipment.  in no particular order -- and this spans the past 30 years:

Magnepan MG-1 speakers
Apt-Holman pre=amp
Technics SL-1200 turntable (the original; just purchased a new one for $1,000)
Sonographe SA-400 amp
Marantz UD-7007 BluRay player (used for SACD's)

Here goes:
 A pair of Celestion  66 Studio Monitors: $5 at a Goodwill Outlet center with one missing stock tweeter that had been replaced with a $2.00 tweeter, but still sound very, very good. I replaced the $2 tweeter with a rare find stock unit (s) ($180 for a pair) and 10 hours of work on the cabinets and grills. They are really an amazing speaker (Abbey Road Studios had used Celestion  66 Studio Monitor for years.
and perhaps better bang for buck for me was:
A pair of Linn Isobarik DMS's I got at a town wide garage sale for $150 with custom Kimber cables, that I listened to for 5 years and am still reluctant to,  sell them.
between the Isobarik's and the Celestions was a pair of Acoustat III 's 
2 more:
Paragon Acoustics "The Regent"  My current speakers  rare find for currently under $1000 but was an accidental purchase keystroke from eBay 
 Infinity Prelude MTS  but the WAF  in our house didn't work.  $10k when new with center channel and a Stereophile Class "A" rating for under $2K and a  Floyd Toole design

OPPO UDP205