Colombo is the city that is closest to the Equator.

Explanation:
The Equator is an imaginary line that divides the Earth into the Northern Hemisphere and the Southern Hemisphere. It is located at 0 degrees latitude.

Colombo is the capital city of Sri Lanka, an island country located in the Indian Ocean. Sri Lanka is situated just off the southern coast of India.

To determine which city is closest to the Equator, we need to look at the latitude of each city. The latitude measures the distance of a location from the Equator. The closer a city's latitude is to 0 degrees, the closer it is to the Equator.

Colombo is located at approximately 6 degrees latitude, which is relatively close to the Equator compared to the other cities listed.

Let's take a look at the latitudes of the other cities mentioned:
- Manila: Manila is the capital city of the Philippines. It is located at approximately 14 degrees latitude, which is further away from the Equator than Colombo.
- Mexico City: Mexico City is the capital city of Mexico. It is located at approximately 19 degrees latitude, which is further away from the Equator than both Colombo and Manila.
- Bangkok: Bangkok is the capital city of Thailand. It is located at approximately 13 degrees latitude, which is further away from the Equator than Colombo but closer than Manila and Mexico City.

Therefore, out of the given cities, Colombo is the closest to the Equator with a latitude of approximately 6 degrees.