I'll go for one that has changed my whole perspective on vinyl replay.
I first started buying records in my teens in the 1970s. As well as the crackles and pops that drove me mad, I always wondered why so many records sounded "harsh" towards side end.
Scroll on many years/decades and joining Forums got me to realise this "harshness" was distortion, inner groove distortion (IGD). Still drove me mad, early Joni Mitchell was unplayable, Tubular Bells, Rumours - all distorted to some degree towards side end.
Then a month or so ago I bought my first micro linear stylus cartridge, an Audio Technica AT-VM95ML, and IGD is no more.
Why has it taken 45 years or so? I have no idea.
So there you have it, my nomination is a micro linear stylus/cartridge.
After that, nominations would be possibly active speakers and a record cleaning machine.