The reactivity trend of the halogens is that the higher up on the Group VIIa column the halogen is, the more reactive it is. Therefore, the most reactive halogen is fluorine, while the least reactive, non-radioactive halogen is iodine.
Halogens are reactive because their outer shells are unfilled and they seek electrons from other elements. Because of this, they are almost never found in a free state.
At standard temperature and pressure, fluorine is a greenish gas. It is so reactive that it even attacks glass, causes water to burst into flame and even forms compounds with noble gases. However, its compounds are often remarkably stable. One of them is Teflon, which is nearly as non-reactive as fluorine is reactive.
Chlorine is also a toxic gas at STP and was famously used as a weapon during World War I. However, when diluted, it's an excellent bleach, bactericide and disinfectant.
Bromine is a rare element that is a liquid at STP. It shares this characteristic with mercury. Bromine has a terrible odor but is used in fire extinguishers. It's also used in sedatives and insecticides.
Iodine is a solid, but when heated, it turns into a violet-colored noxious gas. A tincture of iodine is used to treat wounds. Iodine is used in photography in the form of silver iodide.