Power cords. No question there. If you think a fancy power cord will make the difference, when the remainder of your house wiring is barely equal to a cheap power cord, you are deluding yourself.
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