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. 

Dynaco Stereo 70.  Bought it used for $50.  Used it for nearly 30 years mostly stock, then added a VTA PCB, and new caps and tubes for another hundred bucks or so, and still use it in my main system today as half of a bi-amp situation.
@mikelavigne  - Agreed on the Placette. I had their 4 input passive "pre-amp" in my system for about 3 years. Great sound, very transparent. My buddy still uses one in his system and is very happy with it.
B&K ST140 power amps, Audible Illusions Modulas 3A and B, Shelter 901 series1, Large Advents, Theta DS Pro Prime dac and the Arci Lead Balloon turntable stand, to name a few. Enjoy the music