USGI Poncho

A USGI poncho is recommended.

A poncho is easy to use as quick shelter when forward deployed or as ground cloth.
When you are stationarly for a while in the pouring rain, ponchos have a great deal of utility as well since you can spread them out a bit and generally keep yourself a little dryer.
Make certain your poncho is waterproof. Over time the coatings deteriorate and waterproof performance fails. If your poncho is stinky, that is the tell tail sign that you coating is deteriorated.