Well I've lived in DC and South Florida (Ft. Lauderdale) and have been to Atlanta, and I think I'd pick DC.
If you are not wealthy and beautiful in Miami, then you'll be living in a cookie cutter house, driving for hours on I-95, never walking anywhere and going to a shitload of malls, you'll go to South Beach twice, where you'll blow your entire salary. Nightlife is good though, probably the best out of the three. But, I hated my year there with a passion. The weather sucks half of the year, hot humid and thunderstorms every day in the summer, forget about doing anything outside, although in winter it is real nice, just be prepared to run the AC
24-7 for the other 7 months out of the year. Be prepared for $300.00/month electricity bills (I kid you not) for doing that.
Atlanta - nice city, much more cosmopolitan than its detractors think and it is close to the Appalachians. Doesn't seem to get as disgusting weather wise as the rest of the South, even snows on occasion. if its short term, coming from India, you may select this because it is the most "American" of the three.
DC - the one that feels like a real city, you can take the Metro instead of driving, it's a walkers paradise, it is true a lot of the work is government, but frankly, even though Conservative Republicans are in power, those conservative republicans like to party. I lived there in my twenties at the height of the Reagan years, and believe me it was party, and sex and drugs central. Then we all went to work nursing our hangovers and STDs trying to bring morality back to America
. I hear that has sort of cooled down, but it's still the most urban of cities, and close to Baltimore, Philly, New York, the Shenandoahs, etc., etc.
Probably the most multi cultural too, while Miami is extremely Hispanic, DC has a bit of everything.
For me, I'd choose to live here above the other two.
I may be ugly. I may be an American. But don't call me an ugly American.