A lot of you might disagree, but I say New York City (where I grew up).
Sure, its a terrorist target, but statistically terrorism is very unlikely to kill you.

Advantages:
-Medical care. NYC has some of the best hospitals in the world, and you can get to them quickly compared to somewhere outside of a city where there are long driving times involved in emergency care.

-You don't need to drive. My family doesn't even own a car. Look at the number of people killed in car crashes every year (about 42,000) and suddenly things like terrorism (2,000 7 years ago, once) don't seem like much of a danger. Another 20,000 die each year from the flu... kind of make you wonder if we could make America safer by putting money other places.

-No real natural disasters.

Disadvantages:
-Air pollution
-More crime than rural areas (though not too bad)
-Might be underwater in 100 years
-Terrorism

Anyway, my .02