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. 

About 14 years ago +/- I bought a pair of McIntosh Mc501 mono block amps to drive my 3.6 Maggie’s.  The make the Maggie’s sing as they have the power and current to feed them what they need.  I bought them used, they are still in terrific condition and if I were to sell them (Not!), I could get what I paid all those years ago!

That IS getting your money’s worth out of a product!!
Pair of mint Onix Rocket stand-mount speakers mounted on Pangea stands I picked up for $20. 
Over time I'd say an SME 3009 III on a Micro Seki DX1000 ('78) and my current LS50's. Still can't beat the KEF.
DB Systems db-1A preamp, tone controls and power supply, bought used then upgraded.  
Jeff from Silversmith recommended this with an explanation that the common ground added extra length to the signal path and degraded the sound so a  $2.99 3 to 2 prong ungrounded adapter would provide similar benefits as a high end power cord and that many cords actually  just remove the ground.

I finally dug up an adapter and inserted it between the poser conditioner and the wall.

Worked for me.  It's bigger with a quieter background similar the the change his $1200 speaker cables made on a demo I listened to.  I don't own his speaker cables but will buy him a beer if we meet up again.