what is the most important factor in determining climate

The correct answer is - latitude.

The latitude in many ways is the most important factor in determining the climate of a place. The reason for that is mostly the shape of the planet, which doesn't allow the Sun light to be equally distributed on all of its surface. The lower the latitude, the more direct and more concentrated the Sun light, thus the place will be warmer. The higher the latitude, the sharper the angle of the fall of the Sun light, thus more dispersed Sun light, which will result in a much colder climate.