I'm a big fan of ponchos and liners. Most of the civilian models are pretty thin and weak. Most are pretty clearly considered by manufacturers as cheap, semi-disposable substitutes for a rain suit.

For maximum usability I like the heavily coated nylon USGI models. The older rubberized cotton are good, very tough and adaptable, but heavy. A good one with a liner is a sleeping bag, rain-wear, tarp, tent, body bag, ground cloth, drag, field expedient float, and rainwater collection device.