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. 

Linn AV5140 loudspeakers, originally passive, now activ via Exactbox-i / Charkra / Katalyst with a conservative 100W available for each of 6 drivers.  Purchased in early 2000's.  More lovely today than ever.
The foobar2000 controller PRO app is now free (it might supported haven’t installed it yet)

That’s bang-in

Thanks for tip
Magneplaners, of course.

Love to see the large Advent's mentioned.  Mr. Kloss was a pretty smart guy--I mean, he invented the acoustic suspension speaker...where would be be today without that?  

Unfortunately, those speakers have been passed by in today's world, but KLH and Advent and the Videobeam are clearly major milestones in the industry and Mr. Kloss will always be remembered as the amazing pioneer he was.

Now, as for Jim Winey, well, he simply moved the bar so high that, thus far, no one has figured out how to move it higher.