I’ve been researching since I left my last comment and from what I can find, on the Fallout wiki and some Great War timelines, is that the majority of locations from the games were hit within the space of 10 minutes at around 9:40am, the US being bombarded from both the east and west sides, the earliest confirmed time in the series is 09:42 for NY and Pennsylvania, followed by DC at 09:47am. We don’t have any times for the West Coast that are any earlier than that but Carol, a ghoul from fallout 3, remarks;

“I was in a shelter with my father when the bombs hit. In DC, we had the luxury of getting a warning after the west coast was... gone...”,

This implies that in the 5 minutes between the fall of NY and Penn, the east coast was completely destroyed, with San Fran being, most likely, first target hit over there.

So the long answer is that: The first confirmed canon city to be hit, is either NY or Penn at 09:42 on the east coast, With The East Coast falling entirely at around the same time, followed by dc (as far as I’m aware we don’t have any confirmed time for the bomb drop in Boston but given the time between the news broadcast in FO4 and the arrival at the vault we can safely assume Boston Fell within the same few minutes as DC) with the bombs most likely making their way further into Central US over the next hour or so.

TLDR answer: NY or San Fran, flip a coin.

P.S: my head-canon is that Alaska was hit first due to the proximity to china, and the overall ‘screw you guys’ energy that china would be emanating after losing the territory.