After getting caught in the rain a few times this mild winter and warmish spring, and deliberately riding in the rain to see what works, I'm comfortable with two basic options:

*For cool and cold weather, a waterproof/windproof Shimano Storm Jacket, with appropriate clothes underneath. In cold weather, pretty much anything was comfortable, including cotton t-shirts. In cool weather in the 60s, perspiration tended to make cotton soggy so I preferred cycling style base layer or jerseys.

*In mildly-cool and warm weather, I'd rather just get wet. This spring I've been caught in unexpected rains a couple of times wearing a Pearl Izumi sleeveless baselayer and Garneau short sleeved very thin material jersey. It was remarkably comfortable, even soaking wet, and even when the temp suddenly dipped into the 60s with pretty stout winds. And these fabrics air dry quickly when the rain stops. So I'd be inclined to go with material that isn't waterproof, breathes well and dries quickly, just to stay warm rather than dry.