What do you mean by "best" --- in particular is money a factor here? Cuben fabric is a good choice if not, but for limited use probably overkill (too expensive).

For the specific use you're talking about, I'm wondering if you don't want an oversized tarp, to fit it over your fair weather tent. Just exactly what you have in mind is a factor; put it up as a sort of side wall, or dome it entirely over your other tent? Have trekking poles to hold it up?

You can certainly shave weight with something else on the market. Some standard places to look include mountain laurel designs, integral designs, gossamer gear ...

A good place to look for info on this topic is Mark Verber's site, and in particular http://www.verber.com/mark/outdoors/gear/shelter.html

Personally, I'd be more inclined to get a separate stormy-weather tent and take the appropriate tent along for the particular trip, unless you have some reason to ever use the tarp all by itself.