It really ought to be Detroit.
I know, I know, "Detroit already looks like a post-nuclear wasteland, hurrdurr," but hear me out.
One of the crucial bits of pre-war lore in Fallout is America's "annexation" of Canada.
Detroit is right on the border. Right across the Ambassador Bridge (and the tunnel! talk about a Fallout setting) is Windsor, Ontario, Canada. We can see America's aggression through someone else's eyes for once, and a particular unique perspective at that: no Canadian city is more dependent on America than Windsor.
Moreover, thematically, it makes a lot of sense. So much of the sort of 1950s Americana that Fallout trades in focuses on cars, after all. Why not explore the home of the auto industry? Other bits of Detroit history could be nodded to as well; there could be a raider group that calls itself The Purple Gang, for instance.
Detroit is full of old art deco buildings that would be fun to explore, like the Mall section of Fallout 3. Detroit is also home to an enormous underground salt mine, which would be a great setting.
Finally, the Motown-based radio stations would be the greatest things ever.