My vote would go to one of the possible bomb spawn locations on the default multiplayer map. It spawns on the northwest area of the map. If the bomb explodes, it not only starts a large fire that spreads quickly, but creates a large biohazard area that's guarenteed to infect at least 10 people. This callout gives you way too much to deal with, as you have to block off the area with police, decontaminate with Hazmat engine, and control fires.