Silnylon is really the way to go.  It is essentially impregnated ripstop instead of coated ripstop.  As such it has better waterproof properties and is much more durable and stronger per weight than ordinary ripstop.  But if you what to go whole hog, high tech and have the lightest possible tarp with maximum strength to weight ratio then look into Cuben fibre.  The stuff is amazing (developed for sails for high performance yacht racing) but also the most expensive option.