Hands down it has to be my prescription sunglasses. I need to wear glasses all the time and after years of messing around with clip ons that didn't feel quite right I decides to treat myself and bought a pair of Ray Bans and prescription lenses to go with them. I think it was around $350. 3 years later the frames were broken so I went and bought the same one and had my old lenses put in. I will be investing in a new set soon because the UV coating is all chipped and whatnot. But as a quality of life upgrade for a glasses wearer I struggle to think of anything that has been better.