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. 

Audio Research SP10II. Bought in '85. Used thru 2010. My current Silverline Boleros. Quad 63s USA Monitors and an Threshold SA 3. All great stuff, then and still.
Placette passive Remote Volume Control--replaced a $15k Mark Levinson preamp back in 2001 and sounded better. $1295.

used it for 3 years and loved it in the context of a $250k system.
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.