Best "Bang for the Buck" you have owned?

There are 2 that stand out. Both are amps. The 1st was in 1978, I bought a Harmon Kardon 730 receiver with Original large advents. I never realized how good that thing was until I went to replace it. The Luxman I bought to do so was so anemic, I ended up selling it and repairing the HK which was still going up to about 2000. I powered ut up a while back and was shocked at how BIG the sound was. It will still better a lot of separates today.

The 2nd is a Bob Latino "Dynaco" ST-70. I bought it because it was cheap and I wanted to try a tube amp to see what all the fuss was about. It replaced a $4300 CJ SS amp. That was 10 yrs ago. It STILL in my system today. Looking at my system it is totally outclassed. I just knew that the CJ CT5 preamp was going to show all the flaws of this little amp. Nope. It sounds even better.

Original AR Turntable (1973)
Jolida Glass FX Tube DAC
Tannoy XT 8F Loudspeakers

Recently bought lightly used magnepan 1.7i for $1450. Even at brand new pricing, great bang for buck. Even bigger bang with this deal. Soooo...nice!

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)