Before moving on to the best, let me add my two cents about the worst. Without a doubt, for me nothing comes close to the original Tice power conditioner (don't recall very well anymore, but it was some sort of frozen clock radio -- possibly). I put it into my system and the result was if someone had dulled, squashed, and bleached the music. I returned to the dealer seeking a refund. She refused, say it was a special order. Nothing else I have ever bought comes close to that fraud.
What’s your vote for the most ridiculous, overpriced, and useless tweak?
My vote: Furutech Outlet Cover 105 NCF $220, with “special crystalline material that has two “active” properties.” https://www.thecableco.com/outlet-cover-105-ncf.html