The only thing I have ever found to be very effective in rain is a rain cape. They can make them out of totally waterproof fabric, and you can have the hood up or down. Because they are open around the base, (but still tethered to you so they don't fly up), air can circulate freely under it, keeping you dry. When I am exerting myself (such as when riding), I pump out enough sweat to get drenched under GoreTex, but the circulating air under the rain cape keeps me dry.

The Brits live in rain, they know something about this stuff.