I carry two windproof/waterproof jackets, an REI Novara and a Showerpass Pro.

The Novara is paper thin and light and can be wadded up to fit in a jersey pocket. I wear it when it's cold and windy and drizzling. doesnt' add much sweat.

when it's really pouring though, I pull over and swap that for the Showerspass (which is bulky enough that it has to be kept in a trunk bag).

so far I'm pretty happy