Pakistan's national flag has a dark green field with a vertical white stripe at the hoist, the side nearest the flagpole. A white crescent and five-pointed star appear on the green section, making Pakistan the canonical answer to this question.
The flag was adopted by Pakistan's Constituent Assembly on 11 August 1947, shortly before independence from the British Empire. The green field represents the Muslim majority of the new state, while the white stripe represents religious minorities and their equal place in the country. The crescent is associated with progress, and the star with light and knowledge.
A common mix-up is Iran's flag, which also includes green, white, and red, but has three horizontal bands and a central emblem rather than a white hoist stripe. Saudi Arabia uses a green field too, but its white Arabic inscription and sword are very different. Pakistan's combination of a green field, white hoist panel, crescent, and star is unique among these choices.