What factor most heavily influences the differing climates of Latin America?

Altitude and latitude (proximity to the Equator). For example, Mexico City in Mexico has the same latitude as Hawaii, but has an altitude of 2,420 meters (7,940 feet) above sea level. So, while Hawaii has a mean temperature of 30 °C (80 °F), Mexico City has a mean temperature of just 14 °C (57 °F).