A really good, actually water-resistant raincoat and/or a pair of rain trousers.
For context, I live in the Netherlands where it rains for maybe 3/4 of the year (this is how it feels,anyway) and as I’m on a bike for much of that, a good raincoat or other rain gear is necessary. But the good ones are pretty expensive so I just end up buying the cheap ones, which wear and tear pretty quick, so I guess in the long run I’m spending more money but I can’t avoid it. It’s like the Vime’s Boots theory but with rain gear.