Biggest "bang for the buck" audio gear you have owned(in your history)?

Kingko KA-101 Pro integrated amplifier. Never selling mine. Hard to find as others apparently feel the same way. For both high efficiency speakers and especially headphones.

Kingko KA-101 Pro Tube Integrated Amplifier

I also like the iFi audio Aurora all-in-one system.

iFi audio Aurora

Dyna 70 - $150

A pair of Dyna MK IIIs - $200

Altec Lansing 604Cs - $1500 pair

I still use the Altecs, 43 years.
The Dynas are back up for my Futtermans.

lenco turntable l75

DSPeaker Anti-Mode 2.0 Dual Core room correction unit with additional digital equalization if/as required. For about $850 it was/is a game changer, removing all bass issues from the room and taming a few 'hot-spots' on the frequency spectrum.

I believe once one hears a properly (flat) equalized speaker in a room one will not want to 'bypass' the unit to hear the garbage it removed. Night and Day.  Some eschew this correction; I say enjoy your lumpy bass. My unit will never go back.

Odyssey Audio Kismet stereo amplifier recently replaced a good-sounding W4S STI-1000 integrated. I dropped from 570w to 160w (8 ohm) and the sound of my system was bettered so audibly as to be laughable. Increased pure tonal density, air/bloom, still more clarity, deeper bass, and greater micro-dynamics. I have never been happier with a used audio purchase, ever. Pure dumb luck (research).

I'm going to say my least expensive component,  my Sutherland KC Vibe phono stage makes me think what a great value it is every time I play a record.   It kicks a$$ for the money.    Dead quiet and sounds fantastic.    Textbook example of trickle down that doesn't sound entry level at all.