Why is the Coriolis effect weakest at the Equator?

The Coriolis effect is weakest at the Equator because the magnitude of the horizontal Coriolis force is proportional to the sine of the latitude ($\theta$).

Mathematically, the horizontal component of the force is expressed as:
$$C_f=c(2\omega \sin \theta )$$
where $c$ is the speed of the moving body and $\omega$ is the angular velocity of Earth’s rotation.1

Because the sine of 0° (the latitude at the Equator) is 0, the horizontal deflection becomes zero at that location. While the tangential velocity of a point on Earth is at its maximum at the Equator, the Coriolis effect—which describes the apparent deflection of an object moving within a rotating reference frame—depends on the latitude-dependent change in that velocity relative to the object's path. Consequently, there is no apparent horizontal deflection for ocean currents or other moving bodies at the Equator.12