Covercraft Weathershield or Weathershield HD. The fabric's fibers are treated so the water rolls off the fabric (when the cover is fairly new water from even the hardest thunderstorm will roll off the fabric). I have no moisture-trapping issues; dew will still form on the car naturally, but it's completely gone and dry as quickly as it would be on an uncovered car.
The regular weathershield material is very lightweight and at first glance looks like a $20 nylon cover. After even a <span style="font-style: italic">hard</span> rain two people can shake the water off and roll it up into a small grocery bag size package. It <span style="font-style: italic">will</span> flutter a lot in high winds, but strapping it under the car usually keeps it stable. I always bring one in the Miata or MGB when traveling.
For long-term storage in lots of rain, wind, and sun, the "HD" version is a little better. It's a much heavier fabric so it sits on the car a little better with more stability. I wouldn't hesitate to get either one though.