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. 

Transcendent Sound Grounded Grid kit built preamp.
Dynaco ST70 with original Mullard EL34's
Home built Crites "Cornscala's"
Carver C9 Sonic Hologram Generator $299 in 1982...still using this device now with Magnepans. 
Absolutely turns 2 channel stereo image to a 3 dimensional Soundstage when speakers are placed as the manufacturer recommends. In all the years I have been into audio this was and still is to me a giant killer! Push a button to completely remove it from the circuit and that huge image disappears back to flat 2 channel music, press the button back to holography...its a huge instant sonic upgrade. 
Space tech labs QA112 preamp, made my krell amp sound so much better paired with my apogee duetta 2 speakers.
@invalid  -  Space tech labs QA112 preamp

Noe there's a brand you don't see very often. How did you end up with it? Are you near their shop? I check out their website every once in a while. Their range of product is impressive.