Best "Bang for the Buck" you have owned?

Yes, the CJ ART, various Audio Research were all great...but I do remember lower priced items that gave much pleasure for their price.

In no order, The Large Advent,  DQ-10, Nakamichi SR receivers,  inexpensive Nordost cables, many tuners giving good vibes vis "FREE" music.  Also, in a different way, the Nakamichi TM radios and SoundSpace systems. 

There was a solid state ARC pre that was very

And I do  remember "free" listening at some friends homes/systems that were great values. I was exceedingly lucky to have a best friend that bought most of my gear when I was "moving on".  It gave me the chance to listen to what I had had compared to what I currently least once what I had sold to him was better than what my replacement gear was...........the old "live and learn". 

My purchase in 1991 of a one-owner pair of KLH Nine full-range electrostats for $500! Designed by Arthur Janzen and Roger West, who later started Sound Lab. Found them through a NY Times Sunday classified ad by Hi Fi Classics in Far Rockaway, NY. At the time I was using Quad 57's since 1981. Well, the Nines sounded so much better in bass and dynamic range that they became my main speakers!
The KLH Nines (driven by a Bedini 25/25 and Mogami 2477 speaker cable) had great "boogie factor"! I remember listening to the live Creedence Clearwater LP  and my father coming downstairs and yelling " Turn it down - it's too loud, I can hear it upstairs!" Yes, Dad! Keep on chooglin'!
I have 2

Ric Schultz EVS 1200: class D dual mono 600wp module @ 8 ohms (sadly not being made anymore) $2300

Emerald Physics 3.4: Open Baffle 12' carbon fiber midrange with a concentric one inch polyester tweeter 96dB $4495, but they blew out a small remaining inventory. I saw a pair on USAudiomart for < $900
Two pieces -- 
As edcyn mentioned, my Nola Boxer S1 speakers and the Nelson Pass Amp Camp Amp built from a kit.
Back in 1983 bought a pair of Boston Acoustic 2 way stand mount speakers for next to nothing brand new.  Jaw dropping sound. Wish I had stored them in a box somewhere and saved them. 
There were Saul Marantz, Henry Kloss and other big time people in the audio industry.  Another was Andy Pettit....I believe he was part of early Boston Acoustics. 

1. Total Contact
2. Springs
3. Cable Elevators
4. Bulk Tape eraser
Honorable mention rubber bands, but not fair since they cost nothing. Actually my whole list is not fair, but oh well. Not my fault more people can't think outside the box.
Not equipment, but by far in this hobby: Roon lifetime subscription. $499 some three years ago or so
sonos streamer  200
chord mojo  350
van alstine synergy amp  600
proac tablette 2000 speakers  550
rel sub  400

heaven in second system!!!
Pamela. First semester, freshman year. Only cost me half a joint.

This is so funny! I literally laughed out so hard!
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That would have to be my current Vandersteen 2CE Sig 1’s I purchased for $600.

I just spent $3,750 on my new turntable rig, So it almost seem like they were free In comparison 😁
The first VMPS 626R w/ most of the upgrades for $1200--new territory for sound in my system.  They had tremendous clarity, yet were not hard on the ears, then added a VMPS 215 sub w/ VSS upgrades for just $600.  The sound just got that much better at the mid and even high frequencies in addition to the much better bass.  

Getting the Sistrum SP-3 shelf rack for just $900 used.  Then got 3 RTS couplers to clamp the components to the shelf for just $140.  Each of those used deals increase the live quality of the sound along with more dynamics and clarity.

Each of the PPT products that I had added to the system in the last few years--caused a massive uptake in realism and tone.

NAD 1020 preamp and Snell J-II speakers, both of which I purchased in the late '70s and still use on a regular basis. 
Easily the Clear Day Audio double shotgun speaker cables.  Rest In Peace Paul.
They are fantastic in my system. 
Emotionally, Philips 22 AR470. (full price and some more)

Financially, Technics SL-Q2. (full price)

The only Pamela I have ever known has been as crazy as they get.
Best "Bang for the Buck" you have owned?

Audio wise, A Grundig receiver with matching speakers. Very heavy. Purchased used on C.L. many years ago. Still used on the regular. Giveaway price with original manuals, sales brochures and receipt of purchase from 1970. Quite nice.
Probably my Tekton Pendragons. I auditioned aprox 15-20 pairs of speakers costing upwards of $20K, and then heard Tektons at my buddy’s place. They just sounded right to me while saving me a lot of money. Still have em and still love em after owning them for about 2 yrs now. Zero upgraditis. 
Walked into a local hifi shop in 1976 to check it out. Talked to the owner who had terminal cancer. He only opened the store for his bucket list. Asked him what were his favorite speakers. After spinning some Little Feat I went home with a pair, JansZen Z-410's for $240.
I still use them for the front of the sorround sys with a Raal ribbon in the center.
One transformer and two surrounds in 44 years +$60.
“Honorable mention rubber bands, but not fair since they cost nothing.“

Rubber Band.... Rubber Band.... you’re playing my tune out of tune!!!
Magnepan MGIIIC, clarity, that the wife mentioned upon first listen.  Digital Amplifier Company 4800A.  Marantz AV8801.  Each step up the chain was large increase in SQ, for me.
I have very fond memories of the Apt-Holman preamp that I owned for many years. You youngsters won’t recall it, but was a superb component that represented terrific value back in the late '70s and ’80s. It was designed by Tom Holman, the inventor of 5.1 audio and the inventor of THX motion picture sound.

Read about it here:
zerostat- the red one- bought in the 80's for under $30

Still doing its job, one LP at a time
The original Acurus DIA100. Originally $995 list back in the day. Never one problem, still works and powers my outdoor speakers. Sounds great.
Magneplaners--you owe it to yourself to LISTEN to them in a proper shop

Self-built from kit (1975) Hafler 500 amp--still measures perfectly with minor adjustment to bias three months ago
Infinity Kappa 9 speakers bought in 1988 and the odyssey in finding an adequate power amplifier match ( Ultimately found with bi amping with Metaxas  Solitaires)
But what is with the elastic bands??????????
Dueland DCA16GA interconnect.
Danish tinned copper and oil impregnated cotton dielectric.
By far the best cable I have ever heard and the price is unbelievably low.
These easily replaced my Cardas Golden Presence cables!!
AR14 speakers. Bought new in 1978 for $320. Still sound great today in my youngest son’s dorm room. This is their 3rd tour of duty at the University of TN. 
Chromecast Audio ($15) when Google discontinued them. Magnepan MMG's ($600) - extraordinary for the cash layout.
Best bang for the buck has been my battle cry for the last 30 years after considerable expenditure. To me the question is easy. The Truth line stage. Feed it clean and it will sing!

Raspberry pi 4 with hifiberry digi pro streamer. $60 or so roon endpoint with optical and coax out. Cannot beat it for the price. 

i have fond memories of my acurus dia-100 from the late 80’s... very well built unit, sounded great running a lovely set of spendor sp1/2's i had back then for several years... then i made a mistake and switched to totem 1’s after reading all the glowing reviews... the totems and dia were painfully harsh and sibilant... ugh...
Musical Fidelity m2si integrated amplifier. Definitely a steal at used prices. Really made me start looking at MF products.

Lumin A1 streamer used - sounds is very analog-like and can be had for around $3500 ($8000 new)
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Purchased a used Rotel amp off Ebay, went in to pick it up at a local speaker manufacturer store. Eventually talked myself into some speakers trade for a website I built him (it paid for itself in sales in three weeks). And then managed to get a great job from it. And ultimately made a friend for life and got hooked into proper hifi!

Lucky sod, if your Pamela was half as hot as the couple of Pamelas I’ve known... yeah that was a freakin’ bargain allright.
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In a way, this is a difficult question since, after 50 years of upgrades, I am, thankfully, very satisfied with my system.
However, it is an easy choice... Quad 57s!
I joyfully lived with them for 10 years and miss them to this day.
True, they are frequency limited but they draw you in in a unique and wonderful way. With the complete rebuild, they cost me about $2.5K.

I owned a B&K 202+ amp and still own and use a B&K Pro 10 preamp.I also liked a AMC 2030 tube amp.
I bought my Belles Soloist 1 integrated used (one owner) for $1500 in 2014. It’s never let me down paired with any speakers or source. It’s currently driving ATC SCM 40v2 and has been lovely with other speakers from Maggies to ProAcs...I’d gladly pay the same $1500 for it today!

Yea! I had a B&K 125.2 I picked up and fell in love with, and sold my Adcom 535 without hesitation. They made great sounding equipment, and well worth the money used today. Hated to sell it, but did after I purchased my old Belles 400A. No slouch either, but over twice the cost of the 125.2.

The AR XA table, bought new in ’69 for around eighty bucks, I believe.

My first Decca cartridge, the Blue, new in ’73 for a hundred bucks or so iirc. Changed my life!

QUAD ESL’s/pair of model II power amps/Dahlquist DQ-LP1 x/o, in the early 90’s, all for $800.

Bedini 25/25, $400 in early 90’s.

Townshend Audio Elite Rock table, $650 in early 90’s. Still have, use, and love it.

Eminent Technology LFT-8b loudspeakers, $2499 five years ago. Seriously under-acknowledged and appreciated (in the U.S.A., at least. In the UK it enjoyed a very warm reception. VPI’s Harry Weisfeld also sung it’s praises.).

Electron Kinetics Eagle 2A, $400 a couple of years ago (found here on Audiogon). The best woofer amp ever made? A favorite of JGH, and now I.

Zeta tone arm, $500 just recently (I already had one, but for five hundred bucks?!). Great arm for Deccas/Londons. The Townshend Rock actually came with it’s armboard drilled for the Zeta.

In 1993 while stationed in Germany got a new pair of Klipschorns from the CANEX (Canadian Exchange) for 3999.99 German Mark's, or at the time, just under $2000 CDN. An American Colonel had ordered them while stationed at Frankfurt and was sent back to the US, we were having our annual electronics show and they were about to unbox them when my exwife, who happened to be our Shipper/Receiver with CANEX snagged them for me. She got the 93 slanted Hersey II's out of the divorce, Khorns were to big for her...HA...
For <$600, Vanatoo Transparent 1 Encore active speakers gives anyone with a computer, tablet, or smartphone with streaming software to start with a very flexible and formidable sounding complete system for practically nothing in terms of high end audio $$$$s.