The EuroSCHIRM Swing liteflex trekking umbrella, which has been a part of my gear system for nearly 1½ years. It would probably be the most used piece of gear I own. Unless I’m dead sure that I won’t see any rain on a hike then it is a permanent item on my gear list.
Total rain protection = 395 g which probably weighs less than most traditional rain jackets.
I prefer to pack one of my trekking poles and just hold it instead of a fastening system on my pack. This is also good when the wind gets up and I simply angle the umbrella into the wind. I’ve used it in fairly stormy conditions and it holds up to the task.
It is also UV 50+ rated on the top silver side, so you can use it in heated summer weather as well for shade. I also use my umbrella as shelter when cooking in heaving rain, which I have done on occasions.
I guess I would say that I never leave home without it.