Short term the dollar rallies. In times of crisis its still considered the safe haven. Long run, the fed will have assumed trillions of new obligations and the world will slowly start to drop the dollar as the reserve currency of choice.
Avoid the euro/pound as europe's economy isn't in much better shape. CAD will hold up better, but its only a partial hedge given it's link to the US. Best alternatives are the yen/swiss franc/gold.