what is the busiest fire department in the u.s.a?

Answer 1

Probably FDNY. Here's their statistics for 2005:

Structure fires: 28,455
Non-structure fires: 22,940 (trash, brush, vehicles, etc.)
Non-fire emergencies: 199,643 (usually utility emergencies)
Medical emergencies: 202,526 (fire apparatus sometimes beats EMS to a call)
Malicious fire alarms: 32,138 (where someone pulls a fire alarm where there's no fire, for example, or makes a false report)

Just as a point of interest, the FDNY's average response time is about five minutes, which is really good. New York City has a population of a little over 8 million, so I'm thinking FDNY has the greatest call volume here in the U.S. I was unable to quickly locate similar statistics for LAFD, which might be higher if you add together LA City and LA County.

Answer 2

I WOULD SAY NEW YORK CITY OR LOS ANGELES