My bike. Although that's with the caveat that it's only really effective because I use it all the time for all sorts of things.

Scarpa Hiking boots. 24 years old, 20,000km walked and still worn every week.

Electric toothbrush.

Invest in your teeth. Trust me.

A $50 calf stretcher from Amazon. Rounded bottom, you stand on it and it flexes your feet while extending your heels.

Cured my plantar fasciitis and allowed me to run again.

The good windshield washer fluid. The stuff that's a dollar or two more expensive. Rain basically jumps off my windshield, and I don't need my wipers above 50 unless it's really raining hard. When I actually do use my wipers, they work so much better.

To go along with the mattress theme, and since I’ve had two back surgeries (in my 30s after a car wreck), my sleep number mattress was a big expense but I was miserable and could never get a good night’s sleep before it. It’s been a game changer.
The other is a little more niche, but I’m a runner/endurance sports and my Balega socks have been incredible. They are $14 a pair (yes, $7 a sock), but they are easily worth every single penny. I’ve done 30 hour hiking/running events and my feet were the only thing still dry on my body thanks to those guys.

Hands down a raspberry pi 4. It's a perfect example of why "maintain is more important than obtain" -- what is the point of buying something that I can't "really" use?