It depends. Are you talking about an 80 degree rain? or a 40 degree rain? 80 degree: thin, hooded nylon jacket over a short sleeve cycling shirt. 40 degree: Two layer nylon suit over poly or merino wool base layer (I use Terramar). Keep pedaling and you will stay warm. Stop and you will suffer.
Carry extra base layer and/or shirt in ziploc for after the rain stops or to donn at your turn-around point, because the nylon shell will hold in perspiration.