On the Pacific Northwest, #1 would be a megathrust earthquake off the coastline, followed closely by the eruption of Mount Rainier. The former would produce a magnitude 9 earthquake that would devastate everything west of the Cascades, and an accompanying tsunami that, while it wouldn't do much immediate damage to the region, would flood coastal towns like Coos Bay and Astoria and likely do significant damage in Hawaii. A Rainier eruption, meanwhile, would wipe Tacoma, Washington off the map and cause major ash problems in Seattle and other nearby regions.
For California, I'm gonna go against the conventional wisdom and
not suggest an earthquake. The state is too well-prepared for an earthquake; NorCal had the experience of the 1989 Loma Prieta quake, while SoCal had Northridge in 1994. Instead, I'm gonna suggest a major flood in the Sacramento Valley. The entire region is only protected from flooding by a massive polder system; its failure would result in Stockton and Sacramento looking like the Netherlands in 1954. Take a look at
this map (meant to simulate rising sea levels from melting glaciers, but just as applicable here) and see how far the water would reach into the Central Valley if the polders failed with just
three feet of flooding.
The south's worst-case natural disaster would be a major (category 3+) hurricane hitting Houston, not just for the immediate damage to the area, but for the economic effects. Houston is the heart of the oil industry, home to many,
many refineries that would be taken offline for months, if not years, by a hurricane. It would cause an energy crisis comparable to the '70s oil embargoes. Second place, and also first place for the Midwest by default, would be New Madrid, as so many others have pointed out.
The northeast's worst disaster is plainly obvious: a major hurricane hitting New York. Remember: Sandy was just a Category 1, and it devastated the whole area.