For me, it’s an electric toothbrush. It doesn’t matter if you go with Sonicare or Oral-B, once you start using an electric toothbrush, regular toothbrushes don’t ever feel like they clean your teeth properly. The smooth plaque-free top layer of your teeth that you can feel after using an electric toothbrush can’t be replicated with a regular toothbrush.
This is more of a tool, but I have a set of 4” knipex cobra pliers that are perfect for around the house. Just small enough to fit into about any space, super grippy, and you can easily make minute adjustments to the plier spacing. I’ve used them more than any other tool I own.
Oh, and noise-canceling headphones (ideally over-the-ear). Instant quality of life upgrade.