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

This is very subjective, and your opinion on your own systems/items is all that really counts.

From the beginning to now, these would fit that category for me: (in no order)

Original Large Advents
Dahlquist DQ-10
Conrad Johnson ART pre.
Original Monster interconnects and speaker wires.
Nakamichi SR-3a
AR Turntable
Audio Research LS-12
Magnepan 1.6
Magnepan 20
Wireworld Gold Eclipse III
Audio Technica Electret Condenser headphones 
Magnum Dynalab Etude and 108 tuners
Magnum Dynalab Receiver (original)
And in those "early days" special pressings and imported Jazz albums. 

There may be others I am forgetting, but these were very good items along my journey. 

Yamaha NS 1000's
Dayton Wright XG 10 Mk III's
Dayton Wright LCM1's
Threshold 4000
Citation FM Tuner

The Best Values in Audio? The Starke AD4-320 four channel amp and the Magnepan LRS!
Surprised no one mentioned the Advent 300 with the Tom Holman designed preamp section. $279 back in the 70s.  Great foot in the door to high end:  you could add a power amp (I did with a Marantz8) and used it as a preamp. I kept mine for decades, and used the pre or power sections when my primary gear was out for service, or in between upgrades.
Currently, I am blown away by the magnepan LRS.  I have 25k of electronics driving them, but they still amaze me. I feel like I am in no hurry to find my next “forever” speaker, as these make me happy while I look.