Rain jackets - something breathable. I like to be able to zip it up or down as air and body temps fluctuate during the ride. I use an older patagonia snow shell during the cold weather and a lighter old rei packable rain jacket when it warms up. Neither are completely waterproof. The rei jacket isn't really waterproof at all due to the internal coating being mostly worn off. They both keep me warm (if not damp) when combined with wiking base layers of varying thickness depending on air temps.
A poncho might work well if it had a neck zipper, wasn't too flappy, and was a little breathable. If it is just plastic, perspiration will probably soak from within. In that case, wiking base layers will also be helpful.