People have already said good ones that I strongly agree with, like a cast iron pan, electric toothbrush, and boots. Those were all game changers for me.
It's hard to decide on one that hasn't been said yet, but I'd go with a good water bottle. I've had a bunch over the years since I'll lose them or get new ones for free, and the best are the metal ones that are double walled and keep your water cold. They make it so much easier and convenient to stay hydrated.