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
Conrad Johnson ART pre.
Original Monster interconnects and speaker wires.
ADC XLM II
Audio Research LS-12
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.