Has to be my Yamaha C-80 preamp, for two reasons.
In the last 10 years I've been into using small headphone amps (mostly tube amps) for headphone listening. I got a lot of enjoyment out of that little side-hobby, especially swapping tubes, etc. I got my C-80 as a preamp, never giving a single thought as to how it might perform with headphones. When I ultimately tried it with my headphones, I was absolutely blown away by the quality and power output. All my stand-alone headphone amps were nearly instantly retired. I have had less than stellar experiences using vintage gear for headphones in the past. Of course, many receivers, integrated amps, etc just have their headphone output as a cheap tap off the main speaker amp. That's obviously not the case when talking about a preamp
The preamp has also blown me away just in the sense of how much extra it does for my amps. It allows me to bridge two of my amps that wouldn't otherwise be easily bridgeable. It allows me to invert one channel as it passes through my stereo amps, giving the power supplies extra headroom which helps especially were bass is concerned