Which hurricane/tropical cyclone was the deadliest?

Probably the Bhola cyclone, 1970, Bangladesh, in which 300,000-500,000 people died. It was a category 3 tropical cyclone on the Saffir-Simpson scale.

Originally formed on November 7, 1970, it finally dissipated on November 13, 1970. Damages were estimated at $86.4 million in 1970 US dollars. Three-minute sustained winds were about 185 kilometers per hour (115 miles per hour). The lowest central pressure recorded was 966 millibars (28.53 inches of mercury), the units in which atmospheric pressure is measured. By comparison, normal pressures are typically 1013 millibars (29.92 inches of mercury). The areas most affected were in Bangladesh and India.

Source: NOAA and the WMO